mbedtls: TLS 1.2 premaster sized from dummy ECP_MAX_BITS when p256-m serves P-256 #10

Upstream target: zephyrproject-rtos/zephyr (mbedTLS integration) / Mbed-TLS/mbedtls

With p256-m providing P-256 and no ECP module enabled, mbedTLS sizes the TLS 1.2 ECDHE premaster buffer from a dummy ECP_MAX_BITS of 1 — sizeof(premaster) is literally 1 in the DWARF. psa_raw_key_agreement() then fails with PSA_ERROR_BUFFER_TOO_SMALL, which reaches the application as a bare OSError 138 during the handshake.

Turning p256-m off restores a 32-byte premaster and handshakes complete.

Found while making TLS work on CYW43439 boards; downstream fix in tyeth/circuitpython#21.

Filed here so it can be raised upstream; not yet submitted.