kbuild, kconfig: forcing values from existing .config

[Kevin Hilman <khilman@deeprooted.net>]

I'm looking for the right way to override the settings of an existing
default .config.  Starting from an existing .config (e.g. generated by a
'make defconfig')  I need an automated way of changing specific config
values in order to build a handful of variants based on the same
original .config.

On earlier kernels, I used to just append to the .config, and run 'make
oldconfig' and the appended values would override the existing ones.
This no longer works as 'make oldconfig' now ignores duplicate values.

The allyesconfig/allmodconfig/allnoconfig/randconfig targets allow you
to specify a file containing forced values, but this starts from a clean
slate.  I need this same ability to force specific values, but to use an
existing .config as the starting point instead.

Is there currently a way to do this?  If so, I can't seem to figure out
how.

Kevin