cbootimage: Add 'b' (binary) flag when using fopen to open a binary file.

Add 'b' (binary) flag when using fopen to open a binary file.
This keeps Windows from expanding \n to \r\n and interpreting
<ctrl>z as end of file. The change is to support a Windows
hosted coreboot build environment.

Signed-off-by: Scott Duplichan <scott@notabs.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>