Add auto installation function

Add auto installation function

A new variable AUTO_INSTALL is introduced to kernel cmdline:

* AUTO_INSTALL=n - auto install Android-x86 to the n-th disk or /dev/$n
                   if n is a device node name. If the specified disk
                   doesn't exist, the installer will ask you to select
                   the target disk.

* AUTO_INSTALL=force - auto install Android-x86 to the first disk
                   without any confirmation.

* AUTO_INSTALL=update - auto update Android-x86 to the first partition
                   with label Android-x86, or the first ext4 partition
                   and install boot loader.