apm821xx: remove unnecessary kernel padding

apm821xx: remove unnecessary kernel padding

The allocation of LEBs to ubi volumes is handled by the sysupgrade script:

  package/base-files/files/lib/upgrade/nand.sh

and the ubimkvol and or ubirsvol command. Therefore, padding of the
kernel blob is not needed at all, so use cat instead of dd. The
BLOCKSIZE variable was only used in the dd command.  In any case, 63k
made no sense for the way BLOCKSIZE was being used.

63k (64512) does make sense for DTB_SIZE because of the offsets expected
by u-boot given extant u-boot-env variables.

Tested on Meraki MR24.

Signed-off-by: Russell Senior <russell@personaltelco.net>