reboot-mode: new tool to reboot the device to a specific mode

Very simple tool which does the kernel system call to reboot the device with the additional mode parameter.

This is required because the busybox and musl implementation of the reboot don't have this additional parameter (see https://git.musl-libc.org/cgit/musl/tree/src/linux/reboot.c)

I've tested it only on my Samsung Galaxy Nexus, it would be nice if someone is able to test it on a aarch64 device.

You can read how I've implemented the functionality in the kernel here #175 (comment 181624587)

I'll probably create a subsequent MR to add this tool also in the debug-shell, because it might be useful to reboot to bootloader directly from there instead of booting the whole os

WARNING: the reboot will be instantly, it doesn't do any unmount/sync/... operations