u-boot: add support of *.cfg files

[MBELOUARGA]

Resolve the problem of support of .cfg configuration fragments. When a user adds a fragment.cfg to SRC_URI, it is expecetd that configuration is added to the final image, but it is not done.

[MBELOUARGA]

Hello @noglitch ,

Any news about this MR ?

[gehariprasath]

Hello @MBELOUARGA

Pls allow for sometime to review and decide this amongst our other priorities.

Regards, Hari