flashrom.c: Move read_buf_from_file() to helpers_fileio.c

Constructing a buffer from a read file is auxiliary
functionality to the core flashrom algorithms. Move
aside to decrease the overall complexity of flashrom.c

BUG=b:242246291
TEST=builds

Change-Id: Ia6e1eeb876722899defb5b75346d1f22c70bfbd1
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/66645
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Evan Benn <evanbenn@google.com>
Reviewed-by: Sam McNally <sammc@google.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Thomas Heijligen <src@posteo.de>

meson.build

@@ -91,6 +91,7 @@ srcs = files(
   'flashrom.c',
   'fmap.c',
   'helpers.c',
+  'helpers_fileio.c',
   'ich_descriptors.c',
   'jedec.c',
   'layout.c',