Allow overriding BOOTSTRAP_TOOLSET and only set known ones defaulting to auto-selection

[Flamefire]

BOOTSTRAP_TOOLSET is only determined by B2_TOOLSET if unset, so it can be overriden by users

Additionally it supports "auto" and also defaults to it if B2_TOOLSET has no known mapping to a Bootstrap toolset

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (71c7bb2) to head (0dc1919).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #293   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           22        22           
  Branches        12        12           
=========================================
  Hits            22        22           

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 71c7bb2...0dc1919. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.