uboot-ar71xx: make reproducible

OpenWrt uses ancient u-boot thats not reproducible.

There are multiple upstream changes that introduce
reproducible builds like:
859e92b775fd8ebcfacc591eaf621b677c95b6f7
(not used here - the CMD_DATE/TIMESTAMP functionality
seems to be disabled by config)

70d39f57146a6cb94736db39c770c3d95e07bedb
f3f431a712729a1af94d01bd1bfde17a252ff02c
2d9efa1227262249d381ed5d9d341cbdba76e62d

Instead of changing the Makefile too much
this changeset just tries to use the
changes in Makefile from current upstream git f5fd45f

*Should* fix issue reported by reproducible lede page:
https://tests.reproducible-builds.org/lede/lede.html



Compile tested only
(verified w. hexdump & md5sum)

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>