Refactor libfastboot

This change creates a nice and clean API for issuing
fastboot commands without using the fastboot tool itself.

Test: fastboot tool itself (now using libfastboot2)
on sailfish, walleye, and other devices.
Test: flash bootloader bootloader.img
Test: flash radio radio.img
Test: -w update img.zip
Test: Manually getvar and reboot commands.

Bug: 111126621
Change-Id: I0022536b204ce0c5ad8329367fd522fa3c57877d