mtd: add build hack to reintroduce shared mtd for older releases

mtd: add build hack to reintroduce shared mtd for older releases

When running in SDK context, treat the mtd package as shared to reintroduce
the common repository binary package for older released IBs.

When building outside of the SDK, create a nonshared package with a higher
PKG_REVISION to let opkg prefer that over the shared one in the common repo.

Ref: https://forum.lede-project.org/t/17-01-4-opkg-install-cmd-cannot-install-package-mtd/15312
Fixes: aaac9e82aa ("mtd: mark as nonshared to fix FS#484")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>