kernel: Create recommended configs file

We've started to have more and more "recommended" but not "required"
kernel configs (i.e. hardware-specific, etc). Instead of confusingly
requiring them or forcing them into defconfig files, split these out
into a single file that can be overridden on a per-product basis.

Additionally fixes a bug with the PSTORE config (which needed the
MISC_FILESYSTEMS config too), simplifies the prebuilt check, and
documents some missing target variables.

Bug: 27774946

Change-Id: Ic867aeec8450749c2a471eff1959f544dcaaf804
3 files changed