fboota: Improve adb usage
fboota: Improve adb usage

* Use 'adbro' over 'adb remount'
* 'wait-for-device' before any 'adb' command
* Use common '${adb}'
* 'wait-for-device' before reboot to bootloader
* Always use 'adbsu' in operations inside '/system'
* Add comments to code

Change-Id: Ib3e3cbb4b752f5032d7c876a1352f738ae017640
Signed-off-by: Caio Oliveira <4db4e54f91055ffa8197b8408ec5b5470f9993b8@gmail.com>

AdrianDC/android_development_shell_tools,AdrianDC/android_development_shell_tools