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
diff --git a/wscript b/wscript
index 42532e7..836558e 100644
--- a/wscript
+++ b/wscript
@@ -44,6 +44,8 @@
     conf.env['WITH_TESTS'] = conf.options.with_tests
     conf.env['WITH_TOOLS'] = conf.options.with_tools
 
+    conf.find_program('sh', var='SH', mandatory=True)
+
     conf.check_osx_security(mandatory=False)
 
     conf.check_openssl(mandatory=True)