rework buildafi.get_sim_makevar to work for regular makevars too

previous implementation using '--dry-run --print-data-base' only
worked for 'simple' (i.e. := ) makevars.  Regular (i.e. =) makevars
are not expanded in the data-base dump.  Instead, implemented a phony
'echo' target that takes a manditory ECHOVAR=<varname> parameter
and prints the value of that variable as a line:
<varname> = <value>

to stdout if it exists.  If it is undefined, it prints a message to stderr
and causes make to exit non-zero.

deploy/buildtools/buildafi.py

@@ -40,7 +40,7 @@ def sim_local(cmdline, capture=False):
 def get_sim_makevar(buildconfig, varname):
     """ Dump make database for buildconfig returning value for varname assignment """
     cmdline = "{} | grep -E '^{} :?= '".format(
-        buildconfig.make_recipe('--dry-run --print-data-base'),
+        buildconfig.make_recipe('echo ECHOVAR="{}"'.format(varname)),
         varname
     )
 

sim/Makefile

@@ -115,6 +115,23 @@ test:
 testOnly:
 	cd $(base_dir) && $(SBT_NON_THIN) ";project $(firesim_sbt_project); testOnly $(SCALA_TEST)"
 
+# Phony target for introspecting the final value of a make variable
+# see deploy/buildtools/buildafi.get_sim_makevar for a usage
+.PHONY: echo
+echo:
+ifndef ECHOVAR
+	@echo You must define ECHOVAR=varname for the var you want to print 1>&2
+	@exit 1
+else
+    ifdef $(ECHOVAR)
+	@echo $(ECHOVAR) = $($(ECHOVAR))
+    else
+	@echo Make variable $(ECHOVAR) is not defined 1>&2
+	@exit 1
+    endif
+endif
+
+
 # All target-agnostic firesim recipes are defined here
 include target-agnostic.mk