tests/acceptance/boot_linux: Expand SD card image to power of 2

tests/acceptance/boot_linux: Expand SD card image to power of 2

In few commits we won't allow SD card images with invalid size
(not aligned to a power of 2). Prepare the tests: add the
pow2ceil() and image_pow2ceil_expand() methods and resize the
images (expanding) of the tests using SD cards.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20200713183209.26308-5-f4bug@amsat.org>
(cherry picked from commit 6a289a5ba3383e17fb47029720425bef42e424d7)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>