[build] Accept CROSS= as a synonym for CROSS_COMPILE=

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile

 LDFLAGS		:=
 HOST_CFLAGS	:=
 MAKEDEPS	:= Makefile
+CROSS_COMPILE	?= $(CROSS)
 
 ###############################################################################
 #