build: add buildbot specific config option for setting defaults

build: add buildbot specific config option for setting defaults

This can be used to tweak the buildbot behavior without having to change
buildbot's configuration.
It will also allow us to add more aggressive clean steps (e.g. on
toolchain changes), which would break developers' workflows if enable
by default.

Signed-off-by: Felix Fietkau <[email protected]>