fs: Add a new command to create symbolic links

The command line is:
ln <interface> <dev[:part]> target linkname

Currently symbolic links are supported only in ext4 and only if the option
CMD_EXT4_WRITE is enabled.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>