busybox-initramfs: enhanced init script to allow booting from folder

Adjusts init script in initramfs to allow mounting SYSTEM from a subfolder and /storage too

It introduces 2 new bootoptions, bootfolder (the subfolder on boot where SYSTEM lies) and singlepartition (the subfolder where storage should be mounted)

In essence it mounts the bootdevice in /boot, bindmounts /boot$bootfolder to /flash and if no disk parameter is given bindmounts /boot$singlepartition to /storage, after that the bootprocess is business as usual.

In case singlepartition and disk is given in the commandline, then the normal mountprocess with disk is done.