SDL: disable nasm support

Signed-off-by: Stephan Raue <stephan@openelec.tv>
toolchain: dont build package 'nasm'

Signed-off-by: Stephan Raue <stephan@openelec.tv>
remove package: nasm
scripts/build: cosmetics
scripts/clean: do not clean up more than expected...
create_installstick_osx: enable debug shell in installer
README.md: clean up obsolete ssh_enable instructions
config/version: set OPENELEC_VERSION to 4.1.2

Signed-off-by: Stephan Raue <stephan@openelec.tv>