pk_wrap: reuse a static buffer for signature extraction
pk_wrap: reuse a static buffer for signature extraction

Use a buffer left over after importing a key to hold an extracted signature.
NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls