Synchronize the latest MTK u-boot patches[1]. Some patches have
been amended since last synchronization.
Changes:
* Minor NMBM layer fixes and improvements.
* A new bootmenu shortkey implementation.
* New SPI flash support for en25qx128.
[1] https://github.com/mtk-openwrt/u-boot/tree/mtksoc-20230719
Signed-off-by: Shiji Yang <yangshiji66@qq.com>