initrdscripts: make the kexec script fail hard in unexpected states

[mtoman]

At this moment the kexec initrd script is skipped when ROOTFS_DIR is not defined or if the new rootfs is mounted, but does not contain a kernel image in the expected place. This is undesirable as we assume this is the last script executed by the balena bootloader.

This patch makes the kexec script always execute in the balena bootloader and makes it fail hard in unexpected states, which means the script is always an exit point for the balena bootloader, whether the actual kexec call succeeds or not.

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[mtoman]

@resin-jenkins retest this please

[mtoman]

@resin-jenkins retest this please

[mtoman]

@resin-jenkins retest this please

[jakogut]

@resin-jenkins retest this please

[mtoman]

@resin-jenkins retest this please

[mtoman]

@resin-jenkins retest this please