ubldr: Bump heap size, 1MB -> 2MB

1MB was leaving very little margin in some of the worse-case scenarios with
lualoader. 2MB is still low enough that we shouldn't have any problems with
UBoot-supported boards.

MFC after: 1 week

stand/uboot/common/main.c

@@ -42,7 +42,7 @@ __FBSDID("$FreeBSD$");
 #endif
 
 #ifndef HEAP_SIZE
-#define	HEAP_SIZE	(1 * 1024 * 1024)
+#define	HEAP_SIZE	(2 * 1024 * 1024)
 #endif
 
 struct uboot_devdesc currdev;