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 8639f69a (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>