cmd: fat: add fatmkdir command

In this patch, a new command, fatmkdir, is added.

Please note that, as there is no notion of "current directory" on u-boot,
a directory name specified must contains an absolute directory path as
a parent directory. Otherwise, "/" (root directory) is assumed.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>