build: use new toolchain variables for tools

This change migrates the values of `CC`, `CPP`, `AS` and other toolchain
variables to the new `$(toolchain)-$(tool)` variables, which were
introduced by the toolchain refactor patch. These variables should be
equivalent to the values that they're replacing.

Change-Id: I644fe4ce82ef1894bed129ddb4b6ab94fb04985d
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/plat/st/stm32mp1/cert_create_tbbr.mk b/plat/st/stm32mp1/cert_create_tbbr.mk
index 5b1a3ed..e747e39 100644
--- a/plat/st/stm32mp1/cert_create_tbbr.mk
+++ b/plat/st/stm32mp1/cert_create_tbbr.mk
@@ -11,9 +11,8 @@
 PLAT_INCLUDE	+= -I${PLAT_DIR}include
 
 src/stm32mp1_tbb_cert.o: ${PLAT_DIR}stm32mp1_tbb_cert.c
-	${Q}${HOSTCC} -c ${HOSTCCFLAGS} ${INC_DIR} $< -o $@
+	${Q}$(host-cc) -c ${HOSTCCFLAGS} ${INC_DIR} $< -o $@
 
 PLAT_OBJECTS	= src/stm32mp1_tbb_cert.o
 
 OBJECTS		+= $(PLAT_OBJECTS)
-