Support for other signing methods of ISU packages

Support for other signing methods of ISU packages

As per DA requirement, there is a need to support the new option of
ISU package maker for signing the checksum file in the rpk package.
In addition to the existing '--key <private_key>' method, DA wants to
add the new one: '--sign-cmd <signing command>' option for signing the
checksum file. As soon as this option is added to the isu_pkgs_maker
command line arguments, the following command will be invoked:

    $ ./<signing command> checksum.sha256 checkcsum.sha256.sign

If signing succeeds, the checksum.sha256.sign file is eventually stored
in the rpk package.

Additionally, the PUBKEY variable has been defined in the isu-generator.

Change-Id: Ia8a9078a726705a90b89be8bb7eb085c8af23790