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

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

src/Makefile

@@ -9,6 +9,7 @@ ASFLAGS		:=
 LDFLAGS		:=
 HOST_CFLAGS	:=
 MAKEDEPS	:= Makefile
+CROSS_COMPILE	?= $(CROSS)
 
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