buildroot

doc/manual: document the new waf-package infra variables

Document LIBFOO_WAF_OPTS, LIBFOO_BUILD_OPTS, LIBFOO_INSTALL_STAGING_OPTS
and LIBFOO_INSTALL_TARGET_OPTS.

Also document LIBFOO_CONF_OPTS which was missing in waf-package
reference.

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