package/libubootenv: bump to version f4b9cde

This includes the following changes:

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

And update hash file formatting (2 spaces).

Signed-off-by: Pierre-Jean Texier <pjtexier@koncepto.io>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>