kernel: Better x86 support

- Mention the right kernel image on documentation
- Build dtbs only when the dts folder exists, Google enables CONFIG_OF
  on x86_64 GKI config, this is also the same check done on Linux
  Makefile

Change-Id: I1b0f7a39b0b00f2a4516d2cff79e591136ee3af8
(cherry picked from commit 3fe68ad5)