mtd: fix build against uClibc

Add a rpmatch() implementation when uClibc is missing one.

Fixes:
http://autobuild.buildroot.net/results/171/17187c3c9ba2b9df2bb6ac103e9a54706f07c28b/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>