common: Increase static padding to 8 KiB

In cases, where multible DTBss are included, the kernel FIT image FIT
metadata can grow beyond 4 KiB. Running mkimage with a static external
position of image data of 4 KiB will this cause an error. Increase the
limit to 8 KiB to give enough space for more FIT metadata.

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>