buildroot

mtd: restore installation of libmtd to staging

Commit 67117adc891e06154d23a01b8c8ee348c63e78eb backed out the
deployment of libmtd headers (libmtd.h, libubi.h, ubi-media.h) to
staging and these may be needed by packages that work directly with MTD,
for example swupdate. mtd also produces libmtd.a and libubi.a which are
needed by those packages.

Tested by configuring swupdate to use MTD and verifying that swupdate
now compiles and links.

Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
[Thomas: change to a post-install staging hook, and add comment.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>