build: Avoid passing *FLAGS to sub-make

By default, make won't export CFLAGS defined in the top-level makefile to
the gnu-efi submake. However, if the caller has CFLAGS set in the
environment, it will export the CFLAGS defined in the top-level makefile
to the gnu-efi submake (which it uses due to use of "CFLAGS += ...").

The result is that these two commands both compile but give different
checksums on the final binary:

$ make
$ CFLAGS=not-used make

Avoid this inconsistency by explicitly unexporting the various *FLAGS
from the top-level makefile.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>

Author: rosslagerwall

Makefile

@@ -400,3 +400,4 @@ archive: tag
 
 export ARCH CC CROSS_COMPILE LD OBJCOPY EFI_INCLUDE EFI_INCLUDES OPTIMIZATIONS
 export FEATUREFLAGS WARNFLAGS WERRFLAGS
+unexport CFLAGS CPPFLAGS LDFLAGS