board: samsung: Fix SYS_CONFIG_NAME configs in axy17lte Kconfig

board: samsung: Fix SYS_CONFIG_NAME configs in axy17lte Kconfig

There is a couple of issues related to SYS_CONFIG_NAME config in
axy17lte Kconfig.

1. The global SYS_CONFIG_NAME in axy17lte Kconfig overrides
   SYS_CONFIG_NAME for all boards specified after this line in
   arch/arm/mach-exynos/Kconfig:

       source "board/samsung/axy17lte/Kconfig"

   Right now it's the last 'source' line there, so the issue is not
   reproducible. But once some board is moved or added after this line
   the next build error will happen:

       GEN     include/autoconf.mk.dep
     In file included from ./include/common.h:16:
     include/config.h:3:10: fatal error: configs/exynos78x0-common.h.h:
                            No such file or directory
         3 | #include <configs/exynos78x0-common.h.h>
           |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     compilation terminated.

   That's happening because axy17lte Kconfig defines SYS_CONFIG_NAME
   option in global namespace (not guarded with any "if TARGET_..."), so
   it basically rewrites the correct SYS_CONFIG_NAME defined in the
   hypothetical boards which might appear after axy17lte in mach-exynos
   Kconfig.

2. Another side of the issue is that SYS_CONFIG_NAME is defined
   incorrectly in axy17lte Kconfig:

       config SYS_CONFIG_NAME
           default "exynos78x0-common.h"

   The .h extension should not have been specified there. It's leading
   to a build error, as the generated include file has a double '.h'
   extension.

3. Each target in axy17lte/Kconfig defines its own SYS_CONFIG_NAME. But
   all of those in fact incorrect, as corresponding
   include/configs/<CONFIG_SYS_CONFIG_NAME>.h header files don't exist.

4. The global SYS_CONFIG_NAME pretty much repeats the help description
   from arch/Kconfig and doc/README.kconfig.

Corresponding defconfig files (a*y17lte_defconfig) fix above issues by
overriding SYS_CONFIG_NAME and correctly setting it to
"exynos78x0-common".

Fix all mentioned issues by removing the incorrect global
SYS_CONFIG_NAME and instead specifying it (correctly) in SYS_CONFIG_NAME
options for each target instead.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Fixes: 3e2095e960b4 ("board: samsung: add support for Galaxy A series of 2017 (a5y17lte)")
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>