mbedtls: disable assembly for some scenarios

It's affected in the same way as polarssl, but use an enhanced trick
compared to 130ca81b since it's required by the codebase. Fixes:
http://autobuild.buildroot.net/results/584/584d1d12be9f2bbf45ccbdf93b6b442524059dcc/
http://autobuild.buildroot.net/results/d13/d13a06ba3fc0d7f08a03c04fcb484761da7af3f7/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>