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

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 <daniel@makrotopia.org>