libpayload: Make Kconfig bools use IS_ENABLED()

This will make the code work with the different styles
of Kconfig (emit unset bools vs don't emit unset bools)

Roughly, the patch does this, and a little bit of fixing up:

perl -pi -e 's,ifdef (CONFIG_LP_.+?)\b,if IS_ENABLED\($1\),g' `find . -name *.[ch]`
perl -pi -e 's,ifndef (CONFIG_LP_.+?)\b,if !IS_ENABLED\($1\),g' `find . -name *.[ch]`

Change-Id: Ib8a839b056a1f806a8597052e1b571ea3d18a79f
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/10711
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

payloads/libpayload/libc/time.c

@@ -34,7 +34,7 @@
 
 #include <libpayload-config.h>
 #include <libpayload.h>
-#ifdef CONFIG_LP_ARCH_X86
+#if IS_ENABLED(CONFIG_LP_ARCH_X86)
 #include <arch/rdtsc.h>
 #endif
 
@@ -70,7 +70,7 @@ static void update_clock(void)
 	}
 }
 
-#ifdef CONFIG_LP_NVRAM
+#if IS_ENABLED(CONFIG_LP_NVRAM)
 
 static unsigned int day_of_year(int mon, int day, int year)
 {