site/common: Allow ac_cv_path_SED to be overridden

There are tools in the wild where we need this to be a full path, even
if that doesn't make sense in most other cases due to libtool issues.
Allow those cases to override the default value as currently its near
impossible to do so.

(From OE-Core rev: 65652f419a4c8578121f1f67d43f23ce4eae5a37)

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

meta/site/common

@@ -18,4 +18,4 @@ shadow_cv_passwd_dir=${bindir}
 # python: deactivate a runtime check for ipv6-support in python >=2.7.1 that fails when cross-compiling
 ac_cv_buggy_getaddrinfo=no
 
-ac_cv_path_SED=sed
+ac_cv_path_SED=${ac_cv_path_SED=sed}