lib: Adding the strstr() function

The strstr function finds the first occurrence of a substring
in another string, null terminated strings are not compared, this
function is added for compability for porting other libraries (like
mbedtls)

Jira: ZEP-327

Origin: http://www.leidinger.net/freebsd/dox/libkern/html/d3/d29/


strstr_8c_source.html

Change-Id: I52aac218ce0bd86373ec60f5afc49a92c85f6319
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>