[RFC PATCH SET] build system: update kconfig to bring new features and fixes

[Ron Yorston]

It's probably worth pointing out the patches are nearly a year old.
I started from BusyBox commit 972e29881, otherwise there were problems
applying them.

Not to make it too easy for myself, though, I was working on the BusyBox
port to Microsoft Windows which I maintain[1].  Naturally this has many
tweaks to the build system.

The first three patches went in just fine.  The last two required careful
shepherding but got there in the end.  Cross compiling for Windows on
Linux worked fine, as did a POSIX build on Linux.  I haven't tried a
native compilation on Windows yet.

On the whole, this was less troublesome than I expected.

Cheers,

Ron
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[1] https://frippery.org/busybox