Issue title: Upgrade to latest stable version
 Issue body: I want to upgrade to the latest stable version !
https://github.com/raspberrypi/firmware/blob/stable/extra/uname_string7l
shows: Linux version **4.19.75**-v7l+ 
but apt full-upgrade remains at Linux version **4.19.66** ?
 Comments: 
 Comment 0: See https://www.raspberrypi.org/forums/viewtopic.php?f=66&t=253259&p=1548623#p1548559
 Comment 1: Hi I dont think boot size is the reason - df shows:
/dev/mmcblk0p1  253M     40M  213M   16% /boot
and apt upgrade does not show any errors!


 Comment 2: To clarify, the package is not in the default repo until a few more people have tested it. A normal apt dist-upgrade will not get it right now.
 Comment 3: OK got it, 
I am monitoring dayly the latest stable from https://github.com/raspberrypi/firmware/blob/stable/extra/uname_string7l, 
to find out if a new stable update is available,
so I should monitor the default repo, but where to find it?
{'source': 'GithubIssues', 'repo_name_with_owner': 'raspberrypi/firmware', 'issue_id': 504034324, 'repo_stars': 4689.0}