build-sysroots: add sysroot paths with native binaries to PATH

staging_populate_sysroot_dir() collects postinsts from the sysroot
and executes them. These postinsts, in turn, may call binaries that
are only available from the sysroot. This works fine with recipe-specific
sysroots, as all necessary paths are already in PATH, but breaks down
in this recipe which imitates the old global sysroot way but doesn't adjust
the PATH to include the binary paths from global sysroot.

To reproduce the failure:

$ bitbake docbook-xml-dtd4-native
$ bitbake -c build_native_sysroot build-sysroots

...

Exception: subprocess.CalledProcessError: Command '/home/akanavin/build/tmp/sysroots/x86_64/usr/bin/postinst-docbook-xml-dtd4-native-xmlcatalog' returned non-zero exit status 127.

Subprocess output:
/home/akanavin/build/tmp/sysroots/x86_64/usr/bin/postinst-docbook-xml-dtd4-native-xmlcatalog: 5: /home/akanavin/build/tmp/sysroots/x86_64/usr/bin/postinst-docbook-xml-dtd4-native-xmlcatalog: xmlcatalog: not found
/home/akanavin/build/tmp/sysroots/x86_64/usr/bin/postinst-docbook-xml-dtd4-native-xmlcatalog: 8: /home/akanavin/build/tmp/sysroots/x86_64/usr/bin/postinst-docbook-xml-dtd4-native-xmlcatalog: xmlcatalog: not found

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>