build: break compiler flags out of $(CC)

Having more than the executable in $(CC) only leads to
trouble in a number of situations.

Change-Id: I7642ca4068b3a3bd5798219d74de9e0eb85bb4e5
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/5769
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

toolchain.inc

@@ -25,7 +25,7 @@ ARCHDIR-i386    := x86
 ARCHDIR-x86_32  := x86
 ARCHDIR-armv7   := armv7
 
-CFLAGS_armv7 = \
+CFLAGS_armv7 += \
 	-ffixed-r8\
 	-march=armv7-a\
 	-marm\
@@ -35,7 +35,7 @@ CFLAGS_armv7 = \
 
 toolchain_to_dir = \
 	$(foreach arch,$(ARCH_SUPPORTED),\
-	$(eval CPPFLAGS_$(arch) = \
+	$(eval CPPFLAGS_$(arch) += \
 	-Isrc/arch/$(ARCHDIR-$(arch))/include))
 
 # set_stage_toolchain: Decides the toolchain to be used by every stage