fboots: Improve adb usage
fboots: Improve adb usage

* Use common '${adb}'
* 'wait-for-device' before reboot to bootloader

Change-Id: I2ed6a2dcd590e552bd0a1c58fb8b7e6385b4c4dc
Signed-off-by: Caio Oliveira <4db4e54f91055ffa8197b8408ec5b5470f9993b8@gmail.com>

AdrianDC/android_development_shell_tools,AdrianDC/android_development_shell_tools