[nrf noup] zephyr: Set at least provide EXT_API

-This sets the provide EXT_API to be at least
 optional when the external_crypto is being used.

Ref: NCSDK-12021

Signed-off-by: Georgios Vasilakis <georgios.vasilakis@nordicsemi.no>
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
(cherry picked from commit 5e41a18)
(cherry picked from commit 100c40c)
(cherry picked from commit a2afb37)

boot/zephyr/external_crypto.conf

@@ -18,3 +18,4 @@ CONFIG_SB_CRYPTO_CLIENT_ECDSA_SECP256R1=y
 CONFIG_SB_CRYPTO_CLIENT_SHA256=y
 CONFIG_BL_SHA256_EXT_API_REQUIRED=y
 CONFIG_BL_SECP256R1_EXT_API_REQUIRED=y
+CONFIG_EXT_API_PROVIDE_EXT_API_ATLEAST_OPTIONAL=y