fastboot: Bring back legacy A/B support

Some devices, such as the Essential PH-1, don't have an
updated bootloader and the bootloader is appending an extra
underscore.

This patch works around that issue by sanitizing the slot to be
inline with modern expectations.

Offending commit: https://github.com/aosp-mirror/platform_system_core/commit/42b18a518bac85c3eea14206f6cbafbd1e98a31f

Most of the commit above is not needed because an underscore + slot
is automatically appended when flashing to a partition. Normally, this
would result in something like boot__b instead of boot_b, where b is the
current slot.

Test: flash an image on mata without specifying slot, boots

Change-Id: Ia0b7cee603a4f9ba2e3a61ce6e369ca8c07a7caf