barebox: add an option to embed environment image

barebox: add an option to embed environment image

Barebox provides an option to embed a custom environment image into
barebox binary. This image will be used, when the environment found in
the environment sector is invalid. Both embedded and external images
can be used together, so having both options at the same time is
justified.

This patch sets barebox Kconfig option CONFIG_DEFAULT_ENVIRONMENT_PATH
to user specified path. This way one can use such BR's variables like
BR2_EXTERNAL, TOPDIR etc. to provide paths to custom environment
folders.

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