Use dynamic partition size

64MB is too small when enabling CONFIG_KCOV_ENABLE_COMPARISONS, use
--dynamic_partition_size instead. To align with the new dynamic arg,
previous 64MB comment has been removed too.
Fixes: 391509606
Test: atest --host certify_bootimg_test

Change-Id: I5678e19a051ea99a304f7a70b2a31a552b01520d
Signed-off-by: Jiao, Joey <quic_jiangenj@quicinc.com>