Bump OpenWrt to 19.07.8

[gaby]

• Update OpenWrt to latest 19.07 release
• Merge some of the layers in the Docker image.

Major security fixes with this release:

Security fixes

• Fix FragAttacks (fragmentation and aggregation attacks) vulnerabilities in cfg80211, mac80211, ath10k and ath10k-ct
• We are not sure if some closed source firmware files are still affected by these problems.
• Security Advisory 2021-08-01-1 - XSS via missing input validation of host names displayed (CVE-2021-32019) 19
• Security Advisory 2021-08-01-2 - Stored XSS in hostname UCI variable (CVE-2021-33425) 9
• Security Advisory 2021-08-01-3 - luci-app-ddns: Multiple authenticated RCEs (CVE-2021-28961) 11

Release notes: https://forum.openwrt.org/t/openwrt-19-07-8-service-release/103208

[gaby]

@oofnikj Friendly bump

[gaby]

Once this PR is merge, a separate PR can be created to upgrade to 21.02.0