releasetools: allow overriding partition for boot image

This is required for Asus Transformer devices where the boot image is
not written directly to the boot partition. Instead it is packaged with
a blob header and written to a separate staging partition. The bootloader
moves the boot image to the final location at the next reboot.

Change-Id: Ie97c8f6821c30aa4b3e7c5249fbd98bd9b798ffd