configs/nexbox_a95x_defconfig: bump to kernel 5.10.12

configs/nexbox_a95x_defconfig: bump to kernel 5.10.12

The mmc probing order has changed since commit 21b2cec61c04bd1 (mmc: Set
PROBE_PREFER_ASYNCHRONOUS for drivers that existed in v4.4), so get rid of
the hardcoded root=/dev/mmcblk1p2.  The old vendor U-Boot unfortunately does
not have GPT support, so stick to MBR and use the legacy
root=PARTUUID=<disksignature>-<partition> format and set a fixed disk
signature, similar to how it was done for orangepi-r1 in commit 34cce93adb
(configs/orangepi_r1_defconfig: bump kernel to 5.10.10, u-boot to 2020.10).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>