Draft: u-boot on emmc and /boot on sata

• u-boot-config: enable CONFIG_SATA_BOOT
• patches/0001-tweak-boot-commands.patch: also attempt booting from sata0
• build u-boot image for sd as well as for emmc
• switch to POSIX shell
• set -u
• make shellcheck clean
• avoid cd in favour of -C and env --chdir
• bump u-boot version from v2023.10-rc4 to v2023.10
• verify top git hashes for u-boot and atf
• verify that BPI Block Copy src is correct for sd and emmc
• verify that sd and emmc images differ only in the Block Copy src

marked as draft as we first have to test whether this works

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• 0bc4da6c - patches/0001-tweak-boot-commands.patch: also attempt booting from sata0

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• 04f77a37 - build u-boot image for sd as well as for emmc

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changed title from Draft: u-boot-config: enable CONFIG_SATA_BOOT to Draft: u-boot on emmc and /boot on sata

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• f2f118be - build u-boot image for sd as well as for emmc

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The emmc image that gets built is nonfunctional. Booting it gives the error messge "ERROR: EMMC driver not present. Check your BUILD" from the ATF. Probably there is some dependcy wrong in the Makefile and changing BOOT_MODE does not result in an appropriate rebuild. So better do a clean before rebuilding.