Support out-of-tree build for downstream kernels

This MR:

• modifies pmb/build/menuconfig.py so menuconfig doesn't fail when building out-of-tree
• changes current downstream kernel template a little bit so it compiles the kernel out-of-tree
• adds _outdir to a list of variables to parse in APKBUILD

Closes #1852 (closed) and depends on pmaports!1084 (merged).