build: gate firmware image signing behind SIGN_FIRMWARE

build: gate firmware image signing behind SIGN_FIRMWARE

Firmware image signing in append-metadata/append-gl-metadata was conditional
only by the presence of the build key. Add an explicit SIGN_FIRMWARE option
(default enabled except with BUILDBOT) so appending the fwtool signature can be
toggled on its own.

It's disable on buildbot since we use a decentralized building with no
individual keys on builders. Instead, a fake key is currently used, which adds
an insecure signature.

The future of signing firmware  and key distribution across the build
infrastructure should be discussed separately.

Link: https://github.com/openwrt/openwrt/pull/24291
Signed-off-by: Paul Spooren <mail@aparcar.org>