adb: Add option to reboot into sideload mode in recovery

Currently it requires manual key press to enter the sideload mode. This
CL adds 'adb reboot sideload' to reboot the device into sideload mode
directly with text display on. With 'adb reboot sideload-auto-reboot',
it will reboot after the sideload regardless of the installation result,
unless interrupted by user.

Since it needs to write to /cache/recovery/command file, 'adb root' is
required before calling 'adb reboot sideload' and the one with
'-auto-reboot'.

Also it requires the matching CL in bootable/recovery.

Change-Id: Ib7bd4e216a1efc01e64460659c97c6005bbaec1b