asus-flo: fix using downstream kernel

Don't try to append the dtb if using the downstream kernel.

Fix for:

ERROR: File not found: /usr/share/dtb/qcom-apq8064-asus-nexus7-flo.dtb

Fixes !2239 (merged) ("main/linux-postmarketos-qcom-apq8064: new aport")

Fixes #1553 (closed)