No Linux guru here, so this may be a simple question:

How can I create an SD card image that boots Debian into a shell, or at least enable ssh? I can easily use the canned OS images to create a bootable SD card to reFLASH the eMMC, but I want to be able to use a shell while running the SD card image. Is there a startup file or something I can edit in the canned Debian image on an SD card to prevent it from jumping into the menu to reFLASH the eMMC, and instead simply wait for an ssh connection?

My overall goal is to create an SD card containing an image of Debian fully-configured for my full system configuration. In other words, I want to boot the canned Debian image, make a bunch of configuration changes (installing apps, changing a number of settings, like network settings, etc.), then create a "recovery" SD card that can be used to restore my fully configured OS to the eMMC, should it accidentally get clobbered by, for instance, a power outage. Is there an easy way to do that?