stm32/mboot: Change debug compiler optimisation from -O0 to -Og.

With mboot encrpytion and fsload enabled, the DEBUG build -O0 compiler
settings result in mboot no longer fitting in the 32k sector.  This commit
changes this to -Og which also brings it into line with the regular stm32
build.

ports/stm32/mboot/Makefile

@@ -92,7 +92,7 @@ LDFLAGS += --gc-sections
 # Debugging/Optimization
 ifeq ($(DEBUG), 1)
 CFLAGS += -g -DPENDSV_DEBUG
-COPT = -O0
+COPT = -Og
 else
 COPT += -Os -DNDEBUG
 endif