core: reorder top-level Makefile and document things

This change only moves things around and comments what is done in the
top-level Makefile file, in order to prepare the next changes.

Note that moving the definition of $(O) before or after re-entering make
does not change anything on the buildroot behavior.

This change also renames the variable UMASK to REQ_UMASK.

Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>