[Buildroot] Add description of "make xxxxxx_defconfig" to docs?

[Grant Edwards]

I found the section on saving a configuration for a specific target:

  http://buildroot.uclibc.org/downloads/buildroot.html#board_support

But nowhere does the documentation explain how to use a saved
configuration.After spending some time looking at the Makefile, I
figured out that you simply do a

  $ make  xxxxxxx_defconfig
  $ make
  
[where xxxxxxx_defconfig is the name of any of the files found in
configs/]

Perhaps that's worth a mention in the documentation?

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