When `SOONG_DELVE_STEPS` enviromnment variable is set in addition
to `SOONG_DELVE`, only given soong_build invocations ('steps') are
run in the debugger.
Also, make bootstrapBlueprint code table-driven.

Test: treehugger
Change-Id: Ia4016240ca9e88c2a85c6d06851c5bba30e7b2b5