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 build libxml2 as a host package. Usage 
would be following:

	PKG_BUILD_DEPENDS:=libxml2/host

Signed-off-by: Jiri Slachta <slachta@cesnet.cz>