ramips: sercomm.mk: preparation for Sercomm s1500 devices support

ramips: sercomm.mk: preparation for Sercomm s1500 devices support

This commit moves a part of the code from the "sercomm-factory-cqr" recipe
to the separate "sercomm-mkhash" recipe. This simplifies recipes and
allows insert additional recipes between these code blocks (required for
the future support for Beeline SmartBox PRO router).

dd automatically fills the file by 0x00 if the filesize is less than
offset where we start writing. We drop such dd command so we need to add
--extra-padding-size 0x190 to the sercomm-pid.py call.

Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>