generic: mtdsplit: fit: improve detecting external-data FIT

Currently the detection of external-data FIT images works by checking
if the FIT structure is more than 4 kiB. However, for boards with lots
of different DT-overlays and configurations the FIT structure can
exceed 4 kiB which results in the FIT splitter to fail detecting the
rootfs.
Increase the threshold for external-data FIT to 512 kiB as there aren't
any kernel images smaller than that, and FIT structure less than 512 kiB
will always be an external-data FIT.

Signed-off-by: Daniel Golle <[email protected]>

target/linux/generic/files/drivers/mtd/mtdsplit/mtdsplit_fit.c

@@ -258,7 +258,7 @@ mtdsplit_fit_parse(struct mtd_info *mtd,
 	 * hence we need to parse FDT structure to find the end of the
 	 * last external data refernced.
 	 */
-	if (fit_size > 0x1000) {
+	if (fit_size > 0x80000) {
 		enum mtdsplit_part_type type;
 
 		/* Search for the rootfs partition after the FIT image */