Add add_slot_suffix function.

This function appends androidboot.slot_suffix to the
value of the argument.

Test: apply update
Bug: 153581609
Change-Id: I28a4047b5f2051acc039084f65a71deb492d9dcb
(cherry picked from commit dff80042)
Merged-In: I28a4047b5f2051acc039084f65a71deb492d9dcb