Instead of assuming /sbin contains the correct BusyBox symlinks, directly invoke
the busybox executable. The required utilities are guaranteed to be present,
since the zram-swap package selects them. Additionally, don't assume busybox
resides in /bin, rely on PATH to find it.
While at it, update the copyright year, use SPDX and switch to AUTORELEASE.
Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>