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>

docs/manual/adding-packages-waf.txt

@@ -65,3 +65,21 @@ also be defined.
   default, then Buildroot will use the waf executable provided in the
   package source tree; if set to +YES+, then Buidlroot will download,
   install waf as a host tool and use it to build the package.
+
+* +LIBFOO_WAF_OPTS+, to specify additional options to pass to the
+  +waf+ script at every step of the package build process: configure,
+  build and installation. By default, empty.
+
+* +LIBFOO_CONF_OPTS+, to specify additional options to pass to the
+  +waf+ script for the configuration step. By default, empty.
+
+* +LIBFOO_BUILD_OPTS+, to specify additional options to pass to the
+  +waf+ script during the build step. By default, empty.
+
+* +LIBFOO_INSTALL_STAGING_OPTS+, to specify additional options to pass
+  to the +waf+ script during the staging installation step.  By default,
+  empty.
+
+* +LIBFOO_INSTALL_TARGET_OPTS+, to specify additional options to pass
+  to the +waf+ script during the target installation step.  By default,
+  empty.