Moved nuttx specific make rules to files in makefiles/nuttx.
All target specific makefiles are in their target sub directories.
To minimize file duplication, targets that share rules include a
common file. For example the posix and posix-arm targets both use
makefiles/posix/posix_elf.mk
Signed-off-by: Mark Charlebois <charlebm@gmail.com>
Mark Charlebois