sunxi: add "fel" boot target

This patch makes use of the previous changes to add a new "fel" boot
target for sunxi boards.

When booting via FEL, it's often desirable to work around the absence
of other (usable) boot devices - or to be able to override them,
deviating from the standard boot sequence. To achieve this, the "fel"
boot target gets the highest priority, but won't actually do anything
unless certain criteria are met.

The "bootcmd_fel" implementation proposed here first tests if an actual
FEL boot takes place (using the "fel_booted" env var), and secondly
checks that "fel_scriptaddr" was set (originating from the 'loader',
i.e. the sunxi-tools fel utility). If both checks pass, then it will
try to execute the boot script (boot.scr) at the given address. In case
of an error (e.g. an invalid image), the source command might return
"false", causing "distro_bootcmd" to proceed with the next boot target.

Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>
Acked-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>