librem_mini_v2: Add basic_usb_autoboot build (Release-24-USBAutoboot-3)

Add custom build of PureBoot with basic_usb_autoboot configuration, and
README.md describing custom builds for Mini v2.

Signed-off-by: Jonathon Hall <jonathon.hall@puri.sm>

librem_mini_v2/custom/README.md

+# Custom firmware builds for Librem Mini v2
+
+## pureboot-librem_mini_v2-basic_usb_autoboot
+
+This PureBoot firmware image for the Librem Mini v2 contains:
+* Preconfigured settings for headless boot and recovery
+* Firmware Blob Jail, including Wi-Fi/Bluetooth firmware for Intel AX200 and Bluetooth firmware for Atheros ar3k.
+
+### Headless boot and recovery
+
+The following configuration settings are enabled by default:
+* PureBoot Basic (CONFIG_PUREBOOT_BASIC)
+* PureBoot Basic automatic USB boot (CONFIG_BASIC_USB_AUTOBOOT)
+* Automatically power on after power loss (CONFIG_AUTOMATIC_POWERON)
+
+The PureBoot Basic and automatic USB boot settings are present in the mainline release (off by default) and can be changed from the configuration menu.  They are enabled in this build to support USB recovery boot with no attached display or keyboard.
+
+The automatic power on setting is not in a mainline release yet, but it will be in the next release.  This setting is not yet shown in the configuration menu, but it can be changed by editing config.user manually.  This configures the Mini v2 to automatically power on when power is applied.
+
+### Firmware Blob Jail
+
+The Firmware Blob Jail holds nonfree firmware images required for hardware functionality and provides them to the Linux kernel during boot.  This is needed for Intel AX200 Wi-Fi/Bluetooth cards if the operating system does not provide firmware blobs.  Bluetooth firmware for the Atheros ar3k module is also provided, this card does not require firmware for Wi-Fi.
+
+The blob jail is automatically enabled on devices with Intel AX200, and it can be configured from settings.  The method used to provide firmware to the operating system is currently tailored to Debian-derived distributions and will boot without firmware if the initrd is not compatible.