fastboot: add 'fastboot update -a' argument

Add an argument to accept an alternate filepath for the boot image
when using fastboot update. This lets the user skip the steps of
unzipping a release update.zip and manually flashing system, recovery
partitions etc from the zip file and their debuggable boot image
separately.

Example usage:

    fastboot update -a boot-debuggable.img update.zip

Change-Id: Ic15e1aa271e19b7a61cb783c4ecf94fa4230507f