Add reform-mkimage-hook initramfs hook script

This copies in device-specific files to make them available on all platforms for a universal initramfs that works on all platforms. This is to make sure that the initramfs we create here also contains ls1028a-mhdpfw.bin and /usr/bin/dmesg for LS1028A or /usr/bin/setfont and Uni2-TerminusBold32x16 for the Pocket Reform. Alternatively, we could also add a customize-hook which runs update-initramfs multiple times with different content for /etc/flash-kernel/machine and creates a different initramfs per platform that way. But since we ultimately would like to have an identical /boot independent of the platform, we create a universal initramfs here.