barebox: prepare for auxiliarry build

barebox: prepare for auxiliarry build

No functional changes. In preparation for the auxiliary barebox build,
boot/barebox is split into two parts:
1. boot/barebox:
   - The source and patch specification which are to be shared between the
     barebox and barebox-aux packages.
   - The barebox-package function and build logic.
2. boot/barebox/barebox:
   - The package configuration, fragments, barebox env, etc.
   - The actual barebox package make instantiation.

Signed-off-by: Pieter Smith <pieter@boesman.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>