build: image: posix compatibility cut v head

Replace 2 instances of non posix use of 'head' with posix compliant
'cut'.

'head -c n' cuts 'n' bytes from the passed string and happens to work on
Linux & Mac OS X even though '-c' is not posix.

'head --bytes n' does the same thing and happens to work on linux but
not on Mac OS X and is also not posix.

'cut -b1-8' cuts the first 8 bytes from the passed string and is posix
compliant, hence works on Linux & Mac OS X.

Our usage of 'head --bytes' was particularly unfortunate since it was
used to calculated the RootFS UUID passed to grub - the net result being
a non-functioning system waiting for the root file system to appear.

Thanks to karlp, ynezz & others for pointers on solving this.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

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