Add package signing infrastructure

Add package signing key and certificate configuration options to the
"Image configuration" submenu. If enabled, the Packages.gz list will
be signed as file Packages.sig. The passphrase for the signing key can
be sourced from a file or entered by the user. The signing certificate
is automatically added to the firmware image if opkg-smime is selected.

Signed-off-by: Evan Hunt <each@isc.org>
Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38284 3c298f89-4303-0410-b956-a3cf2f4a3e73