cmd: ums: abort mounting by pressing any key

This patch introduses config which allows interrupt run of usb
mass storage with any key. This is especially useful on devices
with limited input capabilities like tablets and smatphones which
have only gpio keys in direct access.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>