dragon kernel is compressed via lz4 for boot speed and bootloader
support reasons and recent prebuilts happen to include the gzip header
sequence which is causing imgdiff to fail.
Detect a spurious gzip header and treat the section as a normal section.
Bug: 26133184
Change-Id: I369d7d576fd7d2c579c0780fc5c669a5b6ea0d3d
(cherry picked from commit 0f2f6a746af517afca9e5e089a4a17be0a9766d6)
Signed-off-by: David Riley <davidriley@google.com>