FastbootCmds: add oem command to enable mainline support

In order to better support non-android operating systems, such
as postmarketOS (@postmarketOS, https://postmarketos.org/), add
a custom oem command {en,dis}able-mainline-support, which can be
used to toggle different behavior to improve UX when using such
systems.

Right now it does not do anything on its own except printing
whether mainline support is enabled or not when executing
`fastboot oem device-info`.

Change-Id: Icf4323e1402e97a9f81045a5f3225385876e0fce
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>
3 files changed