busybox: don't force FEATURE_NFS_MOUNT

Instead of making the Busybox configuration more complicated by trying
to adjust it depending on whether RPC is available or not (which gets
complicated when RPC support can be provided by libtirpc), simplify
things by letting the user enable FEATURE_NFS_MOUNT or not depending
on whether RPC support is available or not.

Our default configuration do not enable FEATURE_NFS_MOUNT, so users
will not face any build problems by default. Only if they explicitly
enable FEATURE_NFS_MOUNT will they have to make sure that the
toolchain has RPC support, or that libtirpc is enabled (support for
this added in a followup patch).

[Peter: remove from CONFIGURE_CMDS as well]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>