Makefile.inc: Make clang once again a valid toolchain

'prove' that clang is supported (to some extent).

Change-Id: I181f4910ba64ab9746e7ac94aa79da23cdd41dad
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5709
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Tested-by: build bot (Jenkins)
diff --git a/Makefile.inc b/Makefile.inc
index bd1a58a..3205a10 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -29,6 +29,9 @@
 # only run if we're doing a build (not for tests, kconfig, ...)
 ifneq ($(CONFIG_ANY_TOOLCHAIN),y)
 _toolchain=$(shell $(CC_x86_32) -v 2>&1 |grep -q "gcc version .*coreboot toolchain" && echo coreboot)
+ifeq ($(CONFIG_COMPILER_LLVM_CLANG),y)
+_toolchain=coreboot
+endif
 ifneq ($(_toolchain),coreboot)
 $(error Please use the coreboot toolchain (or prove that your toolchain works))
 endif