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/tools/fiptool/plat_fiptool/st/stm32mp1/plat_fiptool.mk b/tools/fiptool/plat_fiptool/st/stm32mp1/plat_fiptool.mk
index 1ba47c1..0d69dbd 100644
--- a/tools/fiptool/plat_fiptool/st/stm32mp1/plat_fiptool.mk
+++ b/tools/fiptool/plat_fiptool/st/stm32mp1/plat_fiptool.mk
@@ -17,7 +17,7 @@
 HOSTCCFLAGS += -DPLAT_DEF_FIP_UUID
 
 ${PLAT_DEF_UUID_FILE_NAME}.o: plat_fiptool/st/stm32mp1/${PLAT_DEF_UUID_FILE_NAME}.c
-	${HOSTCC} -c ${CPPFLAGS} ${HOSTCCFLAGS} ${INCLUDE_PATHS} $< -o $@
+	$(host-cc) -c ${CPPFLAGS} ${HOSTCCFLAGS} ${INCLUDE_PATHS} $< -o $@
 
 PLAT_OBJECTS += ${PLAT_DEF_UUID_FILE_NAME}.o
 endif