Remove LOADER_FDT_SUPPORT as a Makefile variable.

LOADER_FDT_SUPPORT was used inconsistently in the tree. In some
places, it was used to control whether or not the user wanted FDT
included, and in other places it was a command to include
support. Remove it entirely. The former is now enabled -DWITH_FDT,
while the latter is controlled by Makefiles defining HAVE_FDT.

Supported by: Netflix