[package] shell-fm: add libao support

[package] shell-fm: add libao support

- remove madplay dependency
- enable libao support (we already depend on it)
- disable external command in default configuration
- add symlink for .libao
   To configure libao, the library looks for a .libao configuration file in
   the users home directory.
   To allow configuring libao for shell-fm running as a daemon, we symlink
   /etc/libao.conf to the temporary HOME for shell-fm.

[florian: added missing libmad dependency]

Signed-off-by: Christoph Gysin <christoph.gysin@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@31162 3c298f89-4303-0410-b956-a3cf2f4a3e73