src/Kconfig: increase heap size if using flattened image tree

FIT support takes more heap memory than most coreboot payloads.

Change-Id: Id17f25e94d97e937b0e9a9cee3dd1a8aef1d525d
Signed-off-by: Marty E. Plummer <hanetzer@startmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32392
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/Kconfig

@@ -428,6 +428,7 @@ config RTC
 
 config HEAP_SIZE
 	hex
+	default 0x100000 if FLATTENED_DEVICE_TREE
 	default 0x4000
 
 config STACK_SIZE