linux: ensure images/ exist before creating files there

linux: ensure images/ exist before creating files there

When using an initramfs, on the first-pass build, we create a dummy cpio
so that the build succeeeds. The real cpio will come later, and we'll do
a second-pass build to use the actual cpio.

However, when we touch that dummy cpio, the images/ directory may not
yet exist, since commit d0f4f95e39 (Makefile: rework main directory
creation logic) removed its creation at the begining of the build, to
only at the moment we need it, i.e. during the *_INSTALL_IMAGES_CMDS
steps.

However, the linux build is not a _INSTALL_IMAGES_CMDS step, so there is
no guarantee that images/ already exist at that time.

Fix that by explicitly creating images/ before touching the dummy cpio.

Reported-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Matthew Weber <matthew.weber@rockwellcollins.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>