Fix boolean values according to DER specs
Fix boolean values according to DER specs

In BER encoding, any boolean with a non-zero value is considered as
TRUE. However, DER encoding require a value of 255 (0xFF) for TRUE.

This commit makes `mbedtls_asn1_write_bool` function uses `255` instead
of `1` for BOOLEAN values.

With this fix, boolean values are now reconized by OS X keychain (tested
on OS X 10.11).

Fixes #318.

Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls