* only specifically identified artifacts use non-default strategies
  * fallthrough uses default strategies (that will error)
  * adds a custom strategy that prefers a non-rocketchip artifact (notRocketMergeStrategy)
     * needed for chipard/generators/utilities/src/main/resources/csrc/emulator.cc
       so that the ChipYard version is used instead of the one from RC
  * adds (useRocketMergeStrategy) that prefers the RC artifact
     * we want to use vsrc/SimDTM.v from rocketchip, not riscv-sodor

Needed for firesim/firesim#651

common.mk looks for fd but falls back to a slower option using find.
when it isn't found, don't print that it isn't found to the output because
it is okay for it not to be found.

for FireSim chipyard as top, it gets run_scala_main and run_main from chipyard's
variables.mk

It was confusing when I hit it.  A little help without having to go
sift through the scala to find the assertion goes a long way.

…l-script

Requested in https://github.com/ucb-bar/chipyard/pull/872/files#r625522168

not needed in Chipyard, only FireSim