README: update with ci, mali and mainline info
README: update with ci, mali and mainline info

Add more info regarding Docker for u-boot-hardkernel builds
Add Hardkernel wiki info.

Limited odroid-*-hardkernel support. Hard to build via current
ci

Signed-off-by: Armin Kuster <525edcd226fb27e7f47fbb319a38fe989c677548@gmail.com>

akuster/meta-odroid,akuster/meta-odroid,akuster/meta-odroid,akuster/meta-odroid