Log Message:
cups-base: Rename PKG_OPTIONS.cups to PKG_OPTIONS.cups-base

Using a PKG_OPTIONS_VAR that is different to PKGBASE is problematic when
PKG_BUILD_OPTIONS is used. Deprecate the `cups' option (via PKG_LEGACY_OPTIONS
so if `cups' is still used everything will continue to work) in favour of
`cups-base'.

Thanks to <bouyer> for pointing out this problem on tech-pkg@ ML!