libxml2: add host compile

Several packages may depend on host compiled packages during
the build time (such as the new LTS versions of telephony packages).

This commit adds an option to use the libxml2 to be built as
a host package. Usage would be following:

	PKG_BUILD_DEPENDS:=libxml2/host

Signed-off-by: Jiri Slachta <slachta@cesnet.cz>