Fix OSTree branch used for RPi64

OSTree branch installed on RPi 4b boards use
`origin:apertis/v2023dev1/arm64-rpi64/fixedfunction` instead of
`origin:apertis/v2023dev1/arm64-uboot/fixedfunction` . The later is generic
for R-Car and RPi 4b boards.

This commit adds a function to return the expected branch, and adapt it
for the RPi64.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>