adb: Fix the 'adb reboot sideload' for A/B devices.

We used to write the command file (/cache/recovery/command) to trigger
the sideload mode. A/B devices don't support that (may not have /cache
paritition). This CL switches to using libbootloader_message which
writes the command to BCB (bootloader control block) instead.

Test: "adb root && adb reboot sideload" reboots sailfish into recovery
      sideload mode.

Change-Id: I158fd7cbcfa9a5d0609f1f684a2d03675217628f