[crypto] Update cmsdetach to work with python-asn1 version 3.0.0

[crypto] Update cmsdetach to work with python-asn1 version 3.0.0

The python-asn1 documentation indicates that end of file may be
detected either by obtaining a True value from .eof() or by obtaining
a None value from .peek(), but does not mention any way to detect the
end of a constructed tag (rather than the end of the overall file).
We currently use .eof() to detect the end of a constructed tag, based
on the observed behaviour of the library.

The behaviour of .eof() changed between versions 2.8.0 and 3.0.0, such
that .eof() no longer returns True at the end of a constructed tag.

Switch to testing for a None value returned from .peek() to determine
when we have reached the end of a constructed tag, since this works on
both newer and older versions.

Continue to treat .eof() as a necessary but not sufficient condition
for reaching the overall end of file, to maintain compatibility with
older versions.

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