Add Beagle Play support

This series adds Android boot flow support for Beagle Play.

It can be divided in several sections:

1. Misc cleanups: fix broken MMC environment saving + remove unused uboot-env partition
2. Add the missing bits for Beagle Play (eeprom reading/dts index)

Flash procedure:

1. Boot from a DFU bootloader from this codebase. Follow https://baylibre.pages.baylibre.com/ti/android/doc/dfu-recovery.html for detailed documentation.
2. Reset environment:

=> env default -a -f
=> saveenv

3. Program the eMMC as documented previously (if not already done)
4. Switch back to user hw partition:

=> mmc dev 0 0

5. Run fastboot as usual:

=> fastboot usb 0

6. Continue normal flashing procedure.

Refs

• https://software-dl.ti.com/processor-sdk-linux/esd/AM62X/09_00_00_03/exports/docs/linux/Foundational_Components/U-Boot/UG-Memory.html#booting-tiboot3-bin-tispl-bin-and-u-boot-img-from-emmc-boot-partition-for-k3-class-of-socs
• https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1211090/am625-unable-to-reboot-if-using-emmc-boot/4570978

Additional notes

To fully erase the eMMC for AM62x EVM SK, we can run:

# mmc0boot0
=> mmc dev 0 1
=> mmc erase 0 0xfc00

# mmc0boot1
=> mmc dev 0 2
=> mmc erase 0 0xfc00

# user partition
=> mmc dev 0 0
=> mmc erase 0 0x1da4000

For BeaglePlay, do:

=> mmc dev 0 1
=> mmc erase 0 0x2000

=> mmc dev 0 2
=> mmc erase 0 0x2000

=> mmc dev 0 0
=> mmc erase 0 0x1d34000

Dependencies:

• baylibre/ti/android/bootloader/build-bootloaders!26 (merged)
• !29 (merged) (baseline)

TODO

• document https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1211090/am625-unable-to-reboot-if-using-emmc-boot/4570978?tisearch=e2e-sitesearch&keymatch=mmc%20hwreset#4570978
• document https://software-dl.ti.com/processor-sdk-linux/esd/AM62X/09_00_00_03/exports/docs/linux/Foundational_Components/U-Boot/UG-Memory.html#booting-tiboot3-bin-tispl-bin-and-u-boot-img-from-emmc-boot-partition-for-k3-class-of-socs
• rebase on !29 (merged)