wic: bootimg-partition: Add support to create the u-boot boot config file

wic: bootimg-partition: Add support to create the u-boot boot config file

By leveraging the distro boot command feature in the u-boot, we can
compose the corresponding extlinux.conf when creating the wic image,
and let u-boot boot the kernel automatically. For more detail about
the u-boot distro boot command feature, please see doc/README.distro
in u-boot source files.

(From OE-Core rev: bdf8ae540af12ecc9ad60efd3651b0f71d12d3bd)

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>