libubootenv: update to latest git hash

This adds the missing feature 'negative offsets' which was present
in the orginal U-Boot based tools.

git change log:
* f4b9cde Allow negative offsets
* 45bf92a Detect sector size if not found in config
* 9f59db6 uboot_env: remove unused variables
* 65d243e README: libubootenv is now in oe-core

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>