hrm, we'll need to do a bit of prep work on the build side

in the past, we'd run `update_chroot` with specific board names which meant the bot would install+update only the toolchains that board used.  but we changed the builders to run `update_chroot` w/out board info, so now we install+update all default toolchains for all boards now.

the old way allowed us to list new toolchain tuples in boards that didn't have active builders.  the sdk would detect that & build the new target & make binaries available.  then we could update active builders to rely on that toolchain once a sdk run finished.

i think i need to expand the toolchain.conf syntax to add a knob like "don't install it by default, but the sdk bot should build binaries for it".  that way the process would be:
- add new tuple to toolchain.conf w/that sdk setting
- wait for the sdk bot to finish & upload prebuilts
- update toolchain.conf to require the prebuilts

not really that much more work than before.