kconfig: refactor Qt package checks for building qconf

Currently, the necessary package checks for building qconf is
surrounded by ifeq ($(MAKECMDGOALS),xconfig) ... endif.
Then, Make will restart when .tmp_qtcheck is generated.

To simplify the Makefile, move the scripting to a separate file,
and use filechk.  The shell script is executed everytime xconfig
is run, but it is not a costly script.

In the old code, 'pkg-config --exists' only checked Qt5Core / QtCore,
but the set of necessary packages should be checked.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>