docs/manual: document LIBFOO_INSTALL_IMAGES

Document it as are LIBFOO_INSTALL_STAGING and LIBFOO_INSTALL_TARGET.

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

docs/manual/adding-packages-generic.txt

@@ -317,6 +317,11 @@ information is (assuming the package name is +libfoo+) :
   variables are executed to install the package into the target
   directory.
 
+* +LIBFOO_INSTALL_IMAGES+ can be set to +YES+ or +NO+ (default). If
+  set to +YES+, then the commands in the +LIBFOO_INSTALL_IMAGES_CMDS+
+  variable are executed to install the package into the images
+  directory.
+
 * +LIBFOO_CONFIG_SCRIPTS+ lists the names of the files in
   '$(STAGING_DIR)/usr/bin' that need some special fixing to make them
   cross-compiling friendly. Multiple file names separated by space can