Tow-Boot, a new U-Boot option for Pinephone Pro and other devices

[PineFLOWn]

The ideal would be in the SPI a kind of grub where when booting a menu appears that says
Where do you want to start from?
1 eMMC
2 SD

and issue resolved

I'd certainly love to have a Grub like boot screen where for the first 5 seconds on boot I'm presented with three options:

* Power Button = eMMC boot;
* Volume Down = SD boot;
* Volume Up = USB boot;

And if no button is pressed it just boots into the previously selected/default boot option.