sim: testsuite: support basic vars in flags

sim: testsuite: support basic vars in flags

Sometimes in tests, we need supplemental files like linker scripts or
board helper files.  There's no way to set those flags in the tests
currently and relative paths don't work (breaks out of tree builds).

Update the main option parser to replace some strings on the fly.  Now
tests can do things like:

Long term we'll want to switch the framework to use the dejagnu helpers
like dg-xxx that gcc & gdb utilize.  But that'll require more rework.