mkbootimg_test: Generate stronger testdata with random()

Instead of generating zero-filled testdata, use random.seed() and
random.readbytes() to generate testdata with higher entropy.

Bug: 180712476
Test: atest --host mkbootimg_test
Change-Id: I41f32b7059cf4ec2b006791b7a6e98e0a3149345
1 file changed