mach-snapdragon: track boot source

mach-snapdragon: track boot source

Keep track of whether we were loaded via ABL or if U-Boot is running as
a first-stage bootloader.

For now we set this based on if we have a valid external FDT or not,
since it isn't possible to chainload U-Boot from ABL without there being
an external FDT.

This will be used to inform the capsule update logic which partition
U-Boot is flashed to.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
Link: https://lore.kernel.org/r/20250411-b4-qcom-capsule-update-improvements-v2-1-27f6b2fcc4a9@linaro.org
Signed-off-by: Casey Connolly <casey.connolly@linaro.org>