manual: add information about INSTALL_IMAGES_CMDS

The Buildroot manual currently does not contain any mention of the
<pkg>_INSTALL_IMAGES_CMDS which could potentially be of interest when
developing a new 'generic-package'.

Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
CC: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/adding-packages-generic.txt

@@ -410,6 +410,15 @@ different steps of the build process.
   should be installed, since they might be needed to compile other
   packages.
 
+* +LIBFOO_INSTALL_IMAGES_CMDS+ lists the actions to be performed to
+  install the package to the images directory, when the package is a
+  target package. The package must install its files to the directory
+  given by +$(BINARIES_DIR)+. Only files that are binary images (aka
+  images) that do not belong in the +TARGET_DIR+ but are necessary
+  for booting the board should be placed here. For example, a package
+  should utilize this step if it has binaries which would be similar
+  to the kernel image, bootloader or root filesystem images.
+
 * +LIBFOO_INSTALL_INIT_SYSV+ and +LIBFOO_INSTALL_INIT_SYSTEMD+ list the
   actions to install init scripts either for the systemV-like init systems
   (busybox, sysvinit, etc.) or for the systemd units. These commands