package/pkg-waf: properly escape HOST_DIR when defining <pkg>_WAF

package/pkg-waf: properly escape HOST_DIR when defining <pkg>_WAF

When <pkg>_NEEDS_EXTERNAL_WAF is set to YES, <pkg>_WAF is set to
$(HOST_DIR)/bin/waf within the inner-waf-package macro. This reference
to $(HOST_DIR) should use $$(HOST_DIR) so that it is properly expanded
at the time of use, and not at the time of the macro expansion.

In the current Buildroot, this doesn't cause any visible problem
because $(HOST_DIR) points to the same directory for all
packages. However, with per-package host/target directories, this is
no longer the case. It causes a build issue because it tries to use
"waf" from the global host directory, which doesn't exist during the
build.

This commit fixes the following build issue with per package
host/target directories:

/home/test/autobuild/run/instance-2/output/per-package/mpv/host/bin/python2: can't open file '/home/test/autobuild/run/instance-2/output/host/bin/waf': [Errno 2] No such file or directory
make: *** [/home/test/autobuild/run/instance-2/output/build/mpv-0.27.2/.stamp_configured] Error 2

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