bash_completion: Add completion for fastboot.

This CL adds the support for the following:
- serial number completion with '-s';
- slot completion with '--slot';
- argument in 'fastboot flash <partition> <image name>';
- argument in 'fastboot update <image zip>';
- argument in 'fastboot reboot <target>'.

Test: . build/envsetup.sh`; test the above items.
Change-Id: Ib3f6300ac22b1b68e1f92fe28ef091db8467dfa6