huawei-angler: adapt for mainline kernel support (MR 2411)

Now the user can choose to install the proprietary firmware along with
the downstream kernel (linux-huawei-angler) or a mainline kernel
(linux-postmarketos-qcom-msm8994).

Signed-off-by: Petr Vorel <petr.vorel@gmail.com>

[ci:skip-build] Already built on CI in MR