cbfstool: Don't assume compiler is gcc, and use $(CC)

Change-Id: I49feb5be885369fca10c8db31329e51d87031641
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/4841
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

util/cbfstool/Makefile

@@ -1,6 +1,6 @@
 obj ?= $(shell pwd)
 
-HOSTCC ?= gcc
+HOSTCC ?= $(CC)
 CFLAGS ?= -g
 CFLAGS += -D_7ZIP_ST
 CFLAGS += -Wall -Wundef -Wstrict-prototypes -Wmissing-prototypes