Add power off & reboot to bootloader mode in advanced menu

Also:
Cleanup leftovers from previous poweroff option
Always ensure a clean reboot (Thanks to PhilZ for his implementation)

Bootloader option defaults to "bootloader" (fastboot mode), it can be overriden
by defining "ro.bootloader.mode=download" prop for samsung download mode
We leave reboot to bootloader option always enabled, that way we can
override the get_filtered_menu_selection bug that breaks the order
of the options when NULLifying menu options.

Signed-off-by: Michael Bestas <mikeioannina@gmail.com>

Change-Id: I6d674b37ab41ab342b105993719277360b4707e4