build: Introduce HOSTCC flag

Tools are built using the compiler specified in `HOSTCC` instead of
reusing the `CC` variable.  By default, gcc is used.

Change-Id: I83636a375c61f4804b4e80784db9d061fe20af87
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>
diff --git a/tools/fiptool/Makefile b/tools/fiptool/Makefile
index 29eac2a..ee674b7 100644
--- a/tools/fiptool/Makefile
+++ b/tools/fiptool/Makefile
@@ -29,7 +29,7 @@
 
 INCLUDE_PATHS := -I. -I../../include/tools_share
 
-CC := gcc
+HOSTCC ?= gcc
 
 .PHONY: all clean distclean
 
@@ -37,7 +37,7 @@
 
 ${PROJECT}: ${OBJECTS} Makefile
 	@echo "  LD      $@"
-	${Q}${CC} ${OBJECTS} -o $@ ${LDLIBS}
+	${Q}${HOSTCC} ${OBJECTS} -o $@ ${LDLIBS}
 	@${ECHO_BLANK_LINE}
 	@echo "Built $@ successfully"
 	@${ECHO_BLANK_LINE}
@@ -48,7 +48,7 @@
 
 %.o: %.c %.h Makefile
 	@echo "  CC      $<"
-	${Q}${CC} -c ${CPPFLAGS} ${CFLAGS} ${INCLUDE_PATHS} $< -o $@
+	${Q}${HOSTCC} -c ${CPPFLAGS} ${CFLAGS} ${INCLUDE_PATHS} $< -o $@
 
 clean:
 	$(call SHELL_DELETE_ALL, ${PROJECT} ${OBJECTS} fip_create)