uboot: Support multiple environment source files

uboot: Support multiple environment source files

Allow multiple file names to be listed in
BR2_TARGET_UBOOT_ENVIMAGE_SOURCE, concatenating them in the order
listed.

This allows the bulk of the environment to be shared across multiple
boards using a common environment file with board-specific values
supplied in a secondary environment source file.

Signed-off-by: Cam Hutchison <camh@xdna.net>
[Thomas: adjust indentation in the .mk file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>