add APK signing logic

[aparcar]

With this commit it's possible to sign APK package indexes (packages.adb) via the signall.sh script, which is run on the buildmaster. As a consequence apk must be available on the buildmaster. This is the final step to replace OPKG with APK.

[aparcar]

@ynezz tested and works, I'm happy to give you access to the setup I'm running.

[aparcar]

Tested for phase1 and phase2, merging

[ynezz]

As a consequence apk must be available on the buildmaster.

Ok, did you perhaps missed to commit your apk changes for docker/buildmaster/Dockerfile ?

[ynezz]

For some reason this fails https://buildbot.staging.openwrt.org/images/#/builders/1/builds/21/steps/47/logs/stdio:

/scripts/signall.sh /master/signing/mediatek.filogic.tar.gz master

** RUNNING ON BUILDMASTER **
 in dir /master
 argv: ['/scripts/signall.sh', '/master/signing/mediatek.filogic.tar.gz', 'master']
 env: {'CONFIG_INI': '/config/config.ini'}
gpg: keybox '/master/signall.4246/gpg/pubring.kbx' created
gpg: /master/signall.4246/gpg/trustdb.gpg: trustdb created
gpg: key CD84BCED626471F1: public key "OpenWrt Build System (PGP key for unattended snapshot builds) <[email protected]>" imported
gpg: key CD84BCED626471F1: secret key imported
gpg: Total number processed: 1
gpg:               imported: 1
gpg:       secret keys read: 1
gpg:   secret keys imported: 1
find: ‘apk’: No such file or directory
find: ‘apk’: No such file or directory

but the signfiles step is marked as successful, although there is haltOnFailure=True defined for this step, so in this case the build should fail. Probably some issue with exit code handling/passing somewhere?