support/testing/infra: add img_round_power2() function

support/testing/infra: add img_round_power2() function

Since Qemu 5.1, SD card images must have a size that are a power of
two. While some filesystem (such as ext2/3/4) allow to specify the
expected size of the filesystem, others such as SquashFS do not have
this capability.

We were already extending the size of such images to the next 1 MB
boundary using "truncate -s %1M", but that is no longer sufficient. So
instead, we introduce a helper function that extends the size of an
image to the next power of two.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr:
  - use f.trunctate() rather than subprocess.call([truncate,...])
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>