infra/pkg-kconfig: do not update-(def)config to a directory

infra/pkg-kconfig: do not update-(def)config to a directory

Currently, if the user specifies the path to an existing directory as
the path to the custom (def)config file (FOO_KCONFIG_FILE), then we
happily create a file in there, either 'defconfig' or '.config' (or
whatever the .config is named for that package), depending on whether
we're saving a defconfig or a full config.

So, we could save the file, but then the Buildroot defconfig file that
contains that path would no longer be reusable as-is, because we
interpret that path as a path to a file.

Furthermore, if the directory-portion of FOO_KCONFIG_FILE does not
exist yet, the update would fail, because cp does not create missing
directory components.

So we fix that by adding an explicit test for the directory-ness of the
target file, and then an explicit mkdir to create missing directory
components.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>