build: Add missing shell detection

`tools/wrapper/*.sh` assume there is a `SH` variable defined, but the
check was missing.  The detection will be overridden if before running
`./waf configure` a SH environmental variable is defined. For example,

    SH=/usr/local/bin/bash ./waf configure ...

Change-Id: I23fae1630a6b2b7c2083cce9d068af4aaa256cd4