create-spdx: Get SPDX-License-Identifier from source

This patch will read the begining of source files and try to find
the SPDX-License-Identifier to populate the licenseInfoInFiles
field for each source file. This does not populate licenseConcluded
at this time, nor rolls it up to package level.

We read as binary file since some source code seem to have some
binary characters, the license is then converted to ascii strings.

Signed-off-by: Saul Wold <saul.wold@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>