Makefile: define TARGET_DIR_WARNING_FILE relative to TARGET_DIR

Makefile: define TARGET_DIR_WARNING_FILE relative to TARGET_DIR

In commit 7e9870ce32d6329d9e3d602247fbe1709a2275a4 ("core: introduce
intermediate BASE_TARGET_DIR variable"), the definition of
TARGET_DIR_WARNING_FILE was changed to use $(BASE_TARGET_DIR) instead
of $(TARGET_DIR).

However, this change is incompatible with per-package directories, and
is in fact not needed.

With per-package directories, using $(BASE_TARGET_DIR) means that
TARGET_DIR_WARNING_FILE is
output/target/THIS_IS_NOT_YOUR_ROOT_FILESYSTEM. Due to this, when
skeleton-init-common or skeleton-custom attempt to install it, it
fails, because it should be installed to their package per-package
target directory, and not the global output/target directory that doesn't
exist yet. The failure looks like this:

/usr/bin/install -m 0644 support/misc/target-dir-warning.txt /home/thomas/projets/buildroot/output/target/THIS_IS_NOT_YOUR_ROOT_FILESYSTEM
/usr/bin/install: cannot create regular file '/home/thomas/projets/buildroot/output/target/THIS_IS_NOT_YOUR_ROOT_FILESYSTEM': No such file or directory
make[1]: *** [package/pkg-generic.mk:336: /home/thomas/projets/buildroot/output/build/skeleton-init-common/.stamp_target_installed] Error 1

TARGET_DIR_WARNING_FILE is used in three places:

 - In skeleton-custom.mk and skeleton-init-common.mk, where as
   explained above, using $(TARGET_DIR) fixes the use of
   $(TARGET_DIR_WARNING_FILE) in the context of per-package target
   directories.

 - In fs/common.mk, where it is used as argument to $(notdir ...) to
   retrieve just the name of the warning file. So in this case, we
   really don't care about the path of the file, just its name.

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