ports: Convert to use pyexec_file_if_exists() to execute boot/main.py.

The stm32 and nrf ports already had the behaviour that they would first
check if the script exists before executing it, and this patch makes all
other ports work the same way.  This helps when developing apps because
it's hard to tell (when unconditionally trying to execute the scripts) if
the resulting OSError at boot up comes from missing boot.py or main.py, or
from some other error.  And it's not really an error if these scripts don't
exist.

ports/cc3200/mptask.c

@@ -180,7 +180,7 @@ soft_reset:
 
     if (!safeboot) {
         // run boot.py
-        int ret = pyexec_file("boot.py");
+        int ret = pyexec_file_if_exists("boot.py");
         if (ret & PYEXEC_FORCED_EXIT) {
             goto soft_reset_exit;
         }
@@ -205,7 +205,7 @@ soft_reset:
             } else {
                 main_py = mp_obj_str_get_str(MP_STATE_PORT(machine_config_main));
             }
-            int ret = pyexec_file(main_py);
+            int ret = pyexec_file_if_exists(main_py);
             if (ret & PYEXEC_FORCED_EXIT) {
                 goto soft_reset_exit;
             }

ports/esp32/main.c

@@ -111,9 +111,9 @@ soft_reset:
 
     // run boot-up scripts
     pyexec_frozen_module("_boot.py");
-    pyexec_file("boot.py");
+    pyexec_file_if_exists("boot.py");
     if (pyexec_mode_kind == PYEXEC_MODE_FRIENDLY_REPL) {
-        pyexec_file("main.py");
+        pyexec_file_if_exists("main.py");
     }
 
     for (;;) {

ports/esp8266/main.c

@@ -77,9 +77,9 @@ STATIC void mp_reset(void) {
 
     #if MICROPY_MODULE_FROZEN
     pyexec_frozen_module("_boot.py");
-    pyexec_file("boot.py");
+    pyexec_file_if_exists("boot.py");
     if (pyexec_mode_kind == PYEXEC_MODE_FRIENDLY_REPL) {
-        pyexec_file("main.py");
+        pyexec_file_if_exists("main.py");
     }
     #endif
 }

ports/nrf/main.c

@@ -227,12 +227,8 @@ pin_init0();
 
 #if MICROPY_VFS || MICROPY_MBFS
     // run boot.py and main.py if they exist.
-    if (mp_import_stat("boot.py") == MP_IMPORT_STAT_FILE) {
-        pyexec_file("boot.py");
-    }
-    if (mp_import_stat("main.py") == MP_IMPORT_STAT_FILE) {
-        pyexec_file("main.py");
-    }
+    pyexec_file_if_exists("boot.py");
+    pyexec_file_if_exists("main.py");
 #endif
 
     for (;;) {

ports/stm32/main.c

@@ -678,15 +678,12 @@ soft_reset:
     // TODO perhaps have pyb.reboot([bootpy]) function to soft-reboot and execute custom boot.py
     if (reset_mode == 1 || reset_mode == 3) {
         const char *boot_py = "boot.py";
-        mp_import_stat_t stat = mp_import_stat(boot_py);
-        if (stat == MP_IMPORT_STAT_FILE) {
-            int ret = pyexec_file(boot_py);
-            if (ret & PYEXEC_FORCED_EXIT) {
-                goto soft_reset_exit;
-            }
-            if (!ret) {
-                flash_error(4);
-            }
+        int ret = pyexec_file_if_exists(boot_py);
+        if (ret & PYEXEC_FORCED_EXIT) {
+            goto soft_reset_exit;
+        }
+        if (!ret) {
+            flash_error(4);
         }
     }
 
@@ -735,15 +732,12 @@ soft_reset:
         } else {
             main_py = mp_obj_str_get_str(MP_STATE_PORT(pyb_config_main));
         }
-        mp_import_stat_t stat = mp_import_stat(main_py);
-        if (stat == MP_IMPORT_STAT_FILE) {
-            int ret = pyexec_file(main_py);
-            if (ret & PYEXEC_FORCED_EXIT) {
-                goto soft_reset_exit;
-            }
-            if (!ret) {
-                flash_error(3);
-            }
+        int ret = pyexec_file_if_exists(main_py);
+        if (ret & PYEXEC_FORCED_EXIT) {
+            goto soft_reset_exit;
+        }
+        if (!ret) {
+            flash_error(3);
         }
     }
 

ports/teensy/main.c

@@ -302,7 +302,7 @@ soft_reset:
 #if MICROPY_MODULE_FROZEN
     pyexec_frozen_module("boot.py");
 #else
-    if (!pyexec_file("/boot.py")) {
+    if (!pyexec_file_if_exists("/boot.py")) {
         flash_error(4);
     }
 #endif
@@ -322,7 +322,7 @@ soft_reset:
         } else {
             vstr_add_str(vstr, mp_obj_str_get_str(pyb_config_main));
         }
-        if (!pyexec_file(vstr_null_terminated_str(vstr))) {
+        if (!pyexec_file_if_exists(vstr_null_terminated_str(vstr))) {
             flash_error(3);
         }
         vstr_free(vstr);