Dependency on xxd was added in commit c4dd2441 ("tools: add xxd
(from vim)") as U-Boot requires xxd to create the default environment
from an external file.

Later in commit 2b94aac7 ("tools: xxd: use more convenient source
tarball"), xxd from another source was used instead, but that source is
currently unavailable, so let's fix it by using simple xxdi.pl Perl
script instead.

Fixes: #10555
Signed-off-by: NPetr Štetiar <ynezz@true.cz>