initrdscripts: migrate: panic on installation failure

[alexgg]

Right now if the flashing script errors out the initramfs just keeps running modules. This is a security risk specially for secure boot systems as at that point we have an authorized trusted OS running in an unvetted path.

This commit exits init if the flasher returns, and also attemps to crash the kernel followed by an infinite sleep for paranoic reasons.

Change-type: patch

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[mtoman]

lgtm