pygrub: extlinux parsing correctness

pygrub: extlinux parsing correctness

The "in" operator should be used instead of the find method, since
we are only interested in knowing whether the line contains "initrd=",
but we don't care about it's position. Also fixes an error that
happens when initrd= it's at the start of the line, since find returns
0 and is evaluated as False.

Signed-off-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>