Add a new variable to allow markup of postinstall (and preinst)
script dependnecies on native/cross tools.
If your postinstall can execute at rootfs creation time rather than on
target but depends on a native tool in order to execute, you need to
list that tool in PACKAGE_WRITE_DEPENDS.
(From OE-Core rev: aff8ca95b8303a4a2a5600c0d8ec0a50ad677258)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>