Add an implementation for the memchr library function

Add an implementation for the memchr library function

Change-Id: Icded479d246f7cce8a3d2154c69f75178fa513e1
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: http://review.coreboot.org/708
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Mathias Krause <minipli@googlemail.com>