Add the code that performs any required updates to the
/boot/extlinux/extlinux.conf file.  That code is moved over here
from the "rockpro64-post-install" package, which no longer
performs such updates.  This also eliminates any possible issues
arising from the incorrect ordering of packages when they're
installed, which is most important when the images are generated.

While there, improve the produced messages so the users know
exacty what's going on, which should be helpful in debugging any
possible issues, add more comments, and add more different checks
for improved robustness.  Also, use four spaces for indentation
instead of two, for improved distro-wide consistency.

Signed-off-by: Dragan Simic <dsimic@manjaro.org>