Handle short read error correctly in vbutil_kernel.c
Handle short read error correctly in vbutil_kernel.c

If you try to read a file that's all zeros, it tries to read a zero-length
kernel blob, fails to do so (or suceeds with an empty result, depending on
your point of view), and prints

  ERROR: Unable to read kernel blob from <file>: Success

That's not very helpful.

This change makes it say

  ERROR: No kernel blob found

instead.

Change-Id: I841ec6e288f47cd9b1f8e9ca1e6da0741ad20e9f

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/3764004

geekboxzone/mmallow_external_vboot_reference,ionave/seismograph,coreboot/vboot,marineam/seismograph,ccaapton/vboot_reference,marineam/seismograph,ccaapton/vboot_reference,marineam/seismograph,coreboot/vboot,acorn-marvell/vboot_reference,geekboxzone/mmallow_external_vboot_reference,geekboxzone/mmallow_external_vboot_reference,ccaapton/vboot_reference,coreboot/vboot,acorn-marvell/vboot_reference,ccaapton/vboot_reference,geekboxzone/mmallow_external_vboot_reference,geekboxzone/mmallow_external_vboot_reference,coreboot/vboot,ionave/seismograph,coreboot/vboot,ionave/seismograph,acorn-marvell/vboot_reference,marineam/seismograph,acorn-marvell/vboot_reference