Fix use of TARGET_USE_CUSTOM_LUN_FILE_PATH

TARGET_USE_CUSTOM_LUN_FILE_PATH is already available as a
string to extendedcommands.c, regardless of whether it is
quoted or unquoted in BoardConfig.mk.  Removed STRINGIFY define
because it is already available in common.h (along with EXPAND
which shoud be used instead of STRINGIFY anyways).

Change-Id: I3f4ab8d1529df240c200e44049fc60519c0afd5b