package/mtd: install libubi.a and libmtd.a to staging

Currently, the mtd package only installs the mtd utilities into the
target directory. However, the mtd-utils also provide two
static libraries (libmtd.a and libubi.a) that can be used by other
packages or tools.

Therefore, this commit extends the mtd package by adding the
installation to the staging directory of those two libraries.

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>