ubi: Add "skipcheck" command to set/clear this bit in the UBI volume hdr

U-Boot now supports the "skip_check" flag to optionally skip the CRC
check at open time. Currently its only possible to set this bit upon
UBI volume creation. But it might be very useful to also set this bit
on already installed systems (e.g. field upgrade) to make also use of
the boot-time decrease on those systems.

This patch now adds a new "ubi" command "ubi skipcheck" to set or clear
this bit in the UBI volume header:

=> ubi skipcheck rootfs0 on
Setting skip_check on volume rootfs0

BTW: This saves approx. 10 seconds Linux bootup time on a MT7688 based
target with 128MiB of SPI NAND.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>
Cc: Quentin Schulz <quentin.schulz@bootlin.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Andreas Dannenberg <dannenberg@ti.com>