Improve CONV2D coverage for conformance

Enable testing of broadcastable bias channel.
Change convolution arguments function to calculate workable
convolutions by adjusting PAD size as needed for FP and errors.
Add --conv-kernel build_tests option to allow different sized
convolution kernels
Improve coverage with batch size, specific shapes and kernels
of 7x7 and 9x9

Signed-off-by: Jeremy Johnson <jeremy.johnson@arm.com>
Change-Id: I59bb737f398f4363cc606b24fea7812b2059bc01