axs101: add U-Boot support

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
dontmerge: remove custom u-boot tarball

We don't need this archive any more.
Will use mainline archive as soon as new official release of u-boot happens.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
dontmerge: switch to our own sources of u-boot for mkimage tool

As soon as the next official release of upstream u-boot happens ARC architecture
will be built-in, so this patch must be omitted.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>