ext: lib: crypto: Extend generic mbedTLS config with HAVE_ASM

Extend generic mbedTLS configuration file with MBEDTLS_HAVE_ASM option,
to allow the use of assembly code. This improves the performances of
asymetric cryptography, however depending on the architecture and the
CPU, this might have an impact on the code size.

Set the default value ot the previous non-configurable value, ie enable
it by default except on ARM.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>