[yocto] How to specify a machine defconfig for a linux recipe

[Elvis Dowson]

Hi,
       How can I specify a machine defconfig for a linux recipe?

For example, if I set 

# kernel provider
PREFERRED_PROVIDER_virtual/kernel = "linux-omap3"

and try to build my machine, I get the following errors:

 ERROR: Function failed: Fetcher failure for URL: 'file://defconfig'. Unable to fetch URL from any source.
NOTE: package linux-omap3-3.2-r0: task do_fetch: Failed
ERROR: Task 125 (/tool/yocto/meta-gumstix/recipes-kernel/linux/linux-omap3_3.2.bb, do_fetch) failed with exit code '1'
NOTE: Tasks Summary: Attempted 304 tasks of which 302 didn't need to be rerun and 1 failed.

Summary: 1 task failed:
  /tool/yocto/meta-gumstix/recipes-kernel/linux/linux-omap3_3.2.bb, do_fetch


Best regards,

Elvis Dowson