busybox: autodetect nfs mount support based on BR2_INET_RPC setting

Override nfs mount support based on BR2_INET_RPC setting, so BR will DTRT.
This way you will automatically get NFS support in busybox if available in
the toolchain, and the build won't break if the defconfig enables it but
the user has it disabled in the toolchain.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>