Update cells supported for verilog to FIRRTL conversion.

Issue warning messages for missing parameterized modules and attempts to set initial values.
Replace simple "if (cell-type)" with "else if" chain.
Fix FIRRTL shift handling.
Add support for parameterized modules, $shift, $shiftx.
Handle default output file.
Deal with no top module.
Automatically run pmuxtree pass.
Allow EXTRA_FLAGS and SEED parameters to be set in the environment for tests/tools/autotest.mk.
Support FIRRTL regression testing in tests/tools/autotest.sh
Add xfirrtl files to test directories to exclude files from FIRRTL regression tests that are known to fail.

tests/simple/xfirrtl

@@ -0,0 +1,26 @@
+# This file contains the names of verilog files to exclude from verilog to FIRRTL regression tests due to known failures.
+arraycells.v	inst id[0] of
+dff_different_styles.v
+generate.v	combinational loop
+hierdefparam.v	inst id[0] of
+i2c_master_tests.v   $adff
+macros.v	drops modules
+mem2reg.v	drops modules
+mem_arst.v	$adff
+memory.v	$adff
+multiplier.v	inst id[0] of
+muxtree.v	drops modules
+omsp_dbg_uart.v	$adff
+operators.v	$pow
+paramods.v	subfield assignment (bits() <= ...)
+partsel.v	drops modules
+process.v	drops modules
+realexpr.v	drops modules
+scopes.v	original verilog issues ( -x where x isn't declared signed)
+sincos.v	$adff
+specify.v	no code (empty module generates error
+subbytes.v	$adff
+task_func.v	drops modules
+values.v	combinational loop
+vloghammer.v	combinational loop
+wreduce.v	original verilog issues ( -x where x isn't declared signed)