package/rauc: bump to version 1.8

See the release notes[1] on GitHub for changes.

[1] https://github.com/rauc/rauc/releases/tag/v1.8



Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>