bootstrap: improve stage postconditions

In the event of failure to build or execute any step of the bootstrap process, the bootstrap script should be able to resume from the point of failure.

Several checks exist, such as "does this file exist?" and similar.

However they may not accurately reflect the state of the work tree. For example, the mcmtools/sys/tc/native/ directory is assumed to indicate that the native toolchain is built, installed, and working. This is not necessarily true.