compiler.h: add __weak macro

Instead of writing out '__attribute__((weak))' use a shorter form.

Change-Id: If418a1d55052780077febd2d8f2089021f414b91
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/25767
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Justin TerAvest <teravest@chromium.org>

src/security/vboot/vboot_logic.c

@@ -17,6 +17,7 @@
 #include <arch/exception.h>
 #include <assert.h>
 #include <bootmode.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <console/vtxprintf.h>
 #include <delay.h>
@@ -95,21 +96,21 @@ int vb2ex_read_resource(struct vb2_context *ctx,
 }
 
 /* No-op stubs that can be overridden by SoCs with hardware crypto support. */
-__attribute__((weak))
+__weak
 int vb2ex_hwcrypto_digest_init(enum vb2_hash_algorithm hash_alg,
 			       uint32_t data_size)
 {
 	return VB2_ERROR_EX_HWCRYPTO_UNSUPPORTED;
 }
 
-__attribute__((weak))
+__weak
 int vb2ex_hwcrypto_digest_extend(const uint8_t *buf, uint32_t size)
 {
 	BUG();	/* Should never get called if init() returned an error. */
 	return VB2_ERROR_UNKNOWN;
 }
 
-__attribute__((weak))
+__weak
 int vb2ex_hwcrypto_digest_finalize(uint8_t *digest, uint32_t digest_size)
 {
 	BUG();	/* Should never get called if init() returned an error. */