luarocks-package: introduce PKG_NAME_UPSTREAM

luarocks-package: introduce PKG_NAME_UPSTREAM

Since we now require Lua package names to start with "lua", it is likely
that the Buildroot name is different from the upstream LuaRocks name.
Add a feature to the luarocks-package infra that makes it easier to
handle this situation: the package can explicitly specify the upstream
name in PKG_NAME_UPSTREAM, and that name will be used in PKG_ROCKSPEC,
PKG_SOURCE and PKG_SUBDIR.

Add an explanation of this feature to the manual. To make the example
relevant, it is changed to lua-foo, where the upstream name is plain
foo. To avoid confusion with the dependency on a native library, that
dependency is renamed to bar.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>