[comboot] Accept only ".cbt" as an extension for COMBOOT images

COMBOOT images are detected by looking for a ".com" or ".cbt" filename
extension.  There are widely-used files with a ".com" extension, such
as "wdsnbp.com", which are PXE images rather than COMBOOT images.

Avoid false detection of PXE images as COMBOOT images by accepting
only a ".cbt" extension as indicating a COMBOOT image.

Interestingly, this bug has been present for a long time but was
frequently concealed because the filename was truncated to fit the
fixed-length "name" field in struct image.  (PXE binaries ending in
".com" tend to be related to Windows deployment products and so often
use pathnames including backslashes, which iPXE doesn't recognise as a
path separator and so treats as part of a very long filename.)

Commit 1c127a6 ("[image] Simplify image management commands and
internal API") made the image name a variable-length field, and so
exposed this flaw in the COMBOOT image detection algorithm.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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