libtool: Ensure the paths to sed are not hardcoded

libtool: Ensure the paths to sed are not hardcoded

If you:

bitbake sed-native
bitbake libtool-cross

then libtool-cross has SED="/path/to/sysroot/sed" which is incorrect. If that
is reused from sstate or sed-native is cleaned, the build will fail.

This patch simply sets sed to be "sed" since we're not on systems where
the sed from PATH is broken.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>