Allow to work without service

work without bbswitch if prime-select.service is not installed (for suse-prime package)
cleaned up script usage, only displays relevant commands
service, boot and next-boot commands are not available if running without service
handle the case when booting with nvidia config but no nvidia card is available (disabled in BIOS) by forcing intel. Only works if service is running
replaced shell commands using backquotes with more modern $() for consistency
when appropriate, changed nvidia => NVIDIA for consistency
reindented script with emacs for consistency
minor updates to README

openSUSE / SUSEPrime