rp2/boards: Set PICO_BOARD correctly for each board.

In most cases, it's calculated automatically from the board name, and so
doesn't need to be set at all.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

ports/rp2/boards/GARATRONIC_PYBSTICK26_RP2040/mpconfigboard.cmake

 # cmake file
-
-set(PICO_BOARD garatronic_pybstick26_rp2040)

ports/rp2/boards/PICO_W/mpconfigboard.cmake

 # cmake file for Raspberry Pi Pico W
-set(MICROPY_BOARD PICO_W)
-
 set(MICROPY_PY_LWIP ON)
 set(MICROPY_PY_NETWORK_CYW43 ON)
 

ports/rp2/boards/W5100S_EVB_PICO/mpconfigboard.cmake

 # cmake file for Wiznet W5100S-EVB-Pico.
-set(PICO_BOARD pico)
+set(PICO_BOARD wiznet_w5100s_evb_pico)
 set(MICROPY_PY_NETWORK_WIZNET5K W5100S)
 set(MICROPY_PY_LWIP 1)

ports/rp2/boards/W5500_EVB_PICO/mpconfigboard.cmake

 # cmake file for Wiznet W5500-EVB-Pico.
-set(PICO_BOARD pico)
+set(PICO_BOARD wiznet_w5100s_evb_pico)
 set(MICROPY_PY_NETWORK_WIZNET5K W5500)
 set(MICROPY_PY_LWIP 1)