cert_write : fix "Destination buffer is too small" error
cert_write : fix "Destination buffer is too small" error

This commit fixes the `Destination buffer is too small` error returned
by `mbedtls_cert_write` command when the values of `subject_name` or
`issuer_name` parameters exceed 128 characters.

I have increased the size of these varaibles from 128 to 256 characters,
but I don't know if it's the best way to solve this issue...

Fixes #315.

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