Add makefile rule for linux-defconfig

[r2d4]

I find myself referencing this documentation every time I make kernel
configuration changes. I've added a simple rule to open the kernel
configuration menu, and then save your changes to the right directory.

[codecov-io]

Codecov Report

Merging #1896 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1896   +/-   ##
=======================================
  Coverage   32.23%   32.23%           
=======================================
  Files          69       69           
  Lines        4104     4104           
=======================================
  Hits         1323     1323           
  Misses       2608     2608           
  Partials      173      173

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