xen: Update Kconfig to Linux v5.4

xen: Update Kconfig to Linux v5.4

This patch updates Kconfig to a more recent version of Kconfig, found
in Linux v5.4.0, 219d54332a09 ("Linux 5.4").

With the updated version of Kconfig, other changes are necessary to
avoid breaking the build.

Kconfig files:
- fix Kconfig files that where using option env=*:
  Since Linux commit 104daea149c4 ("kconfig: reference environment
  variables directly and remove 'option env='"), we can access the
  environment directly via $() and "option env=" as been removed.
- CONFIG_EXPERT='y' will now appear in .config file if
  XEN_CONFIG_EXPERT=y in the environment. The alternative is to change
  "EXPERT" to "$(XEN_CONFIG_EXPERT)" in all Kconfig files.

Makefile:
- silentoldconfig target as been removed from Kconfig. To update
  include/generated/autoconf.h, we need to use syncconfig target
  instead.

Makefile.kconfig:
- Import newer needed code from Linux's Makefile.lib and
  Kbuild.include and Makefile.build.
- Set Q to empty, Xen build system doesn't silence commands. Having Q
  empty mean we can import stuff from Linux without having to remove the
  leading $(Q) from build commands. And quiet='' means commands will be
  echoed.
- Add $(PHONY) to .PHONY. Like it is intended by Kbuild.

Makefile.host is also updated and copied from Linux.

Dependency change:
- Now depends on flex/bison, maybe we could _shipped those files like
  before. Linux doesn't do that anymore.

The .gitignore in kconfig/ has more entries, compared to upstream, for
file generated by Makefile.host.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>