[package] [splash-osmc] fix variable substitution
[package] common.sh: build_in_env: mount procfs silently as may already be mounted; umount /mnt silently as may already be bound from dirty build; use /usr/bin/make not /bin/make
[toolchains] umount /mnt prerm just in case we have a dirty build
[toolchains] fix postinst and prerm rules to work for new package conventions