intelvbttool: Add support for reading vbt from sysfs

VBT on Intel(R) systems is available via sysfs as
    /sys/kernel/debug/dri/0/i915_vbt
However the size of this file reads as 0 causing
intelvbttool to fail. This patch implements incremental reads
with realloc for such cases or whenever the file size is not
available (e.g. reading from stdin).
After this patch is applied, intelvbttool can be used as follows:
    sudo intelvbttool -f /sys/kernel/debug/dri/0/i915_vbt -d

Change-Id: I5d17095a5747550b7115a54a7619b7294a846196
Signed-off-by: Alex Feinman <alexfeinman@hotmail.com>
Reviewed-on: https://review.coreboot.org/c/31531
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

0 files changed, 0 insertions, 0 deletions