kconfiglib: update with 'imply' support

Corresponds to 375506d (File writing nit) from upstream
(https://github.com/ulfalizer/Kconfiglib).

Adds proper 'imply' support and fixes a few minor issues, one of which
previously triggered the following weird warning:

  configs/taurus_defconfig: /tmp/tmpisI45S:6: warning: assignment to SPL_LDSCRIPT changes mode of containing choice from "arch/$(ARCH)/cpu/u-boot-spl.lds" to "y"

The change in 8639f69 (genconfig.py: Print defconfig next to warnings)
was reapplied.

tools/moveconfig.py previously depended on a hack that merged 'select's
with 'imply's. It was modified to look at the union of
Symbol.get_selected_symbols() and Symbol.get_implied_symbols(), which
should give the same behavior.

tools/genboardscfg.py was verified to produce identical board.cfg's
before and after the change.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
diff --git a/tools/moveconfig.py b/tools/moveconfig.py
index 6f549a5..e311646 100755
--- a/tools/moveconfig.py
+++ b/tools/moveconfig.py
@@ -1472,7 +1472,7 @@
     """
     sym = kconf.get_symbol(imply_config)
     if sym:
-        for sel in sym.get_selected_symbols():
+        for sel in sym.get_selected_symbols() | sym.get_implied_symbols():
             if sel.get_name() == config:
                 return sym
     return None