bootstrap: Read configuration from config.mk

bootstrap: Read configuration from config.mk

During the transition period where we're still using ./configure and makefiles,
read some extra configuration from `config.mk` if it's present. This means that
the bootstrap build should be configured the same as the original ./configure
invocation.

Eventually this will all be removed in favor of only storing information in
`config.toml` (e.g. the configure script will generate config.toml), but for now
this should suffice.