[mb2] refactor deploy to remove duplicated code

Signed-off-by: Juha Kallioinen <juha.kallioinen@jolla.com>
[mb2] use _skip_install_processing macro

When rsync deploy (run_install) is called, rpmbuild will
define _skip_install_processing macro, which makes rpmbuild
skip the packaging specific checks making deploy a bit faster.

Signed-off-by: Juha Kallioinen <juha.kallioinen@jolla.com>