unix/Makefile: And note why btree module is disabled for coverage build.

unix/Makefile

@@ -263,6 +263,8 @@ freedos:
 	MICROPY_PY_JNI=0
 
 # build an interpreter for coverage testing and do the testing
+# btree module is disabled, because with extra warnings enabled,
+# building berkeleydb errors out
 coverage:
 	$(MAKE) COPT="-O0" MICROPY_PY_BTREE=0 CFLAGS_EXTRA='-DMP_CONFIGFILE="<mpconfigport_coverage.h>" -fprofile-arcs -ftest-coverage -Wdouble-promotion -Wformat -Wmissing-declarations -Wmissing-prototypes -Wold-style-definition -Wpointer-arith -Wshadow -Wsign-compare -Wuninitialized -Wunused-parameter -DMICROPY_UNIX_COVERAGE' LDFLAGS_EXTRA='-fprofile-arcs -ftest-coverage' BUILD=build-coverage PROG=micropython_coverage