core/pkg-kconfig: ensure kconfig base and fragment files exist

Even though we do have a dependency chain back to each of the kconfig
base and fragment files:

    $$($(2)_DIR)/.config: $$($(2)_KCONFIG_FILE) $$($(2)_KCONFIG_FRAGMENT_FILES)

we can't rely on it to ensure they are all present, because they all have
this rule:

    $$($(2)_KCONFIG_FILE) $$($(2)_KCONFIG_FRAGMENT_FILES): | $(1)-patch

but since this rule has no prerequisite (only build-order, but that does
not count in this case) and no recipe, make will believe each missing
file to be a PHONY target, and will always run targets that depend on
it:
    https://www.gnu.org/software/make/manual/make.html#Force-Targets



So, that means a missing kconfig base or fragment file would always
cause the rule to generate .config to be run at each invocation, which
in turn would cause a rebuild of the kernel, which is clearly not what
we want.

Since this is expected make behaviour, we can well end up with a missing
Kconfig base or fragment. To avoid continuously rebuilding the kernel in
that case, we must check those files exist by ourselves, and error out
if any one of them is missing.

One would expect we check for them right in their dependency rule, like
so:

    $$($(2)_KCONFIG_FILE) $$($(2)_KCONFIG_FRAGMENT_FILES): | $(1)-patch
        [ -f $(@) ] || {echo Missing $(@) >&2; exit 1; }

but that does not work, as only the first target is tested for. That
check msut be turned into a loop explicitly testing all files, like so:

    $$($(2)_KCONFIG_FILE) $$($(2)_KCONFIG_FRAGMENT_FILES): | $(1)-patch
        for f in $$($(2)_KCONFIG_FILE) $$($(2)_KCONFIG_FRAGMENT_FILES); do \
            [ -f $(@) ] || {echo Missing $$$${f} >&2; exit 1; }; \
        done

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Floris Bos <bos@je-eigen-domein.nl>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>