[boot] uboot-lantiq: switch to release tarballs from github

Directly download release tarballs created from signed git tags on
https://github.com/danielschwierzeck/u-boot-lantiq. Also delete
obsolete patch files.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
[boot] uboot-lantiq: add packages for Lantiq EASY50712

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>