boot/afboot-stm32: create destination directory during installation

boot/afboot-stm32: create destination directory during installation

Following d0f4f95e390bcb1c953efa125f5277a8a235396e ("Makefile: rework
main directory creation logic"), BINARIES_DIR is not implicitly
created by the main Makefile at the beginning of the build, leaving
that up to whatever piece of code needs to install something in
$(BINARIES_DIR).

The afboot-stm32 package didn't pay attention to this, which this
commit fixes.

While at it, we move the afboot-stm32 installation into
<pkg>_INSTALL_IMAGES_CMDS, because using <pkg>_INSTALL_TARGET_CMDS to
install only files to BINARIES_DIR is a bit strange.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/131217111

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>