main/postmarketos-initramfs: add function to source deviceinfo (MR 4037)

As a preparation for #1836, move the sourcing of deviceinfo to its own
function. This should allow to have a single point in the whole of the
initramfs for the sourcing of the deviceinfo, instead of every script
that needs it sourcing it. This allows to change the way deviceinfo is
sourced in the future with a lot less effort.