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If you forget to do this, memory allocations on the heap probably won't work and your user tasks won't start.

To see why, tools/Config.mk assigns a value to KDEFINE such that the preprocessor symbol __KERNEL__ will be defined when certain source files are compiled. KDEFINE is passed to nested invocations of 'make' as EXTRAFLAGS. If your board's scripts/Make.defs still attempts to use EXTRADEFINES, the preprocessor symbol __KERNEL__ will not be defined in some of the places that it should be. Suppose you're building a FLAT build. In this case, include/nuttx/mm/mm.h will not define MM_KERNEL_USRHEAP_INIT like it should, which will cause nx_start.c not to call up_allocate_heap() at startup. Therefore, any attempt to allocate memory on the heap will fail.

Rename src/Makefile to src/Make.defs and Modify

Rename If your custom board directory is in a board family that uses a 'boards/ARCH/FAMILY/common' directory (such as boards/arm/stm32/common, boards/arm/cxd56xx/common, etc), then you'll need to make two minor changes to your custom board's src/Makefile:

(1) Rename it from src/Makefile to src/Make.defs., and

(2) Near the end of that file, replace this line, which usually appears at the end:

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If you forget to do this, 'make' will report an error, "no rule to make libboard.a," and the build will fail.

Major Changes to the Build System

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