I would avoid the SPI flash. Having a good U-Boot on the eMMC works. It won't let you boot multiple operating systems on the same media, but it will let you boot from any device. You easily flash a good U-Boot over the Manjaru one without affecting Manjaru itself.
The one that I use which comes with NetBSD (and can probably be sourced other ways) doesn't have a "menu" but it does have a command interpreter that you can invoke by pressing a key when prompted.
See this: https://youtube.com/shorts/jVab9bcjM_I
The one that I use which comes with NetBSD (and can probably be sourced other ways) doesn't have a "menu" but it does have a command interpreter that you can invoke by pressing a key when prompted.
See this: https://youtube.com/shorts/jVab9bcjM_I
:wq
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[ SRA accepts you ]