grub recipe is machine specific

[fboudra]

Per comments on #141
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hmm. this looks a bit hacky.. if the config files are specific to hikey they should be in SRC_URI only for hikey. or maybe we should use COMPATIBLE_MACHINE. either way this doesn't look good on non Hikey platforms..
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and
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Side note, SRC_URI won't work, it will leave cfg empty and will fail in grub-mkimage. So if we want to fix the machine specific bits, COMPATIBLE_MACHINE might be the right approach.
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