src/lib: add IS_ENABLED() around Kconfig symbol references

Some of these can be changed from #if to if(), but that will happen
in a follow-on commmit.

Change-Id: Idcea3f8b1a4246cb6b29999a84a191a3133e5c78
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20341
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/lib/ramtest.c

@@ -6,7 +6,7 @@ static void write_phys(unsigned long addr, u32 value)
 {
 	// Assembler in lib/ is very ugly. But we properly guarded
 	// it so let's obey this one for now
-#if CONFIG_SSE2
+#if IS_ENABLED(CONFIG_SSE2)
 	asm volatile(
 		"movnti %1, (%0)"
 		: /* outputs */
@@ -31,7 +31,7 @@ static u32 read_phys(unsigned long addr)
 
 static void phys_memory_barrier(void)
 {
-#if CONFIG_SSE2
+#if IS_ENABLED(CONFIG_SSE2)
 	// Needed for movnti
 	asm volatile (
 		"sfence"