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>