buildsystem: check for coreboot toolchain by default

Other toolchains just don't cut it.

Change-Id: I7a0bdf60d89b5166c9a22c9e9f3f326b28f777b8
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/4584
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/Kconfig

@@ -70,6 +70,16 @@ config COMPILER_LLVM_CLANG
 
 endchoice
 
+config ANY_TOOLCHAIN
+	bool "Allow building with any toolchain"
+	default n
+	depends on COMPILER_GCC
+	help
+	  Many toolchains break when building coreboot since it uses quite
+	  unusual linker features. Unless developers explicitely request it,
+	  we'll have to assume that they use their distro compiler by mistake.
+	  Make sure that using patched compilers is a conscious decision.
+
 config SCANBUILD_ENABLE
 	bool "Build with scan-build for static code analysis"
 	default n