src/context: read file or compatible string for variant-{dtb,file}
src/context: read file or compatible string for variant-{dtb,file}

When setting in your system configuration

  [system]
  variant-dtb=true
  ...

or

  [system]
  variant-file=/path/to/file
  ...

this will trigger the context setup method to fetch the variant string
from eithe the content of the given file or from the compatible string
of the running system.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

rauc/rauc,jluebbe/rauc,jluebbe/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc,rauc/rauc,ejoerns/rauc,ejoerns/rauc