librem5 devkit: resurrect device support

This MR does:

• Updates kernel to 5.2-based kernel from Purism
• Updates mesa-purism-gc7000 to Mesa 19.1 + Purism patches
• Updates u-boot, ATF, and uuu to exact versions used by Purism for the PureOS images
• Builds the m4 firmware binary (instead of relying on prebuilt binary from Purism)
• Fixes some issues in and simplifies the uboot script for booting