Combining kernel configs?

[Ralphred]

So, it's that time of the decade when hardware gets "handed down" like a Mexican Wave, previous iterations of this decennial ritual have taught me how to "normalise" CFLAGS/CPUFLAGS and do an emerge -e world.
I can just install two kernels, one for the old hardware and one for the new (all *my* kernels are monolithic), or just make a -dist kernel work for the swap over, but I'm interested in the concept of combining two .config files.
My intuition says to cat one into the other, remove all the "is not set" lines, sanity check the non-tristate (non-boolean in my case) entries, and pass the whole mess through sort -u before running make oldconfig - there must be a more elegant way to do this that doesn't involve the "fun" of an extensive make menuconfig session, someone must have done this before?

[NeddySeagoon]

Ralphred,

Free with the kernel source tree is

[pingtoo]

You can try

[Ralphred]

Thanks both, make oldconfig <some other config> does nothing I'm afraid.

./scripts/diffconfig is cool, and I managed to get some sensible output from it; I also underestimated the vast difference between the two .config files, some 1400 differences, and ~170 options that needed "turning on" on the new kernel for it to stand in for the old one.
Needless to say I'm not that into fondling a kernel config that will only be useful for literally two boot cycles, and I'm off to copy a kernel that I know won't boot the old hardware. I plan to use the grub menu to avoid it until it's needed. I've got about 10 years to author a script for "next time" though