UEFI: Conditionally define the ASSERT macro

Only define the ASSERT macro when it is not already defined.  This
change allows the UEFI/FSP definitions to be included with most other
coreboot includes.

BRANCH=none
BUG=None
TEST=Build and run on sklrvp

Change-Id: Iccfeb83eb1e52623ae0a0fe2a96b587ce61f82d7
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: http://review.coreboot.org/10334
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/vendorcode/intel/edk2/uefi_2.4/uefi_types.h

@@ -66,12 +66,14 @@ static inline void debug_dead_loop(void)
 }
 
 #define _ASSERT(expression)  debug_dead_loop()
+#ifndef ASSERT
 #define ASSERT(expression)			\
 	do {					\
 		if (!(expression)) {		\
 			_ASSERT(expression);	\
 		}				\
 	} while (FALSE)
+#endif
 
 //
 // Contents of the PEI_GRAPHICS_INFO_HOB