boot: Rename boot_enc_decrypt to boot_decrypt_key

All of boot_enc_ function follow the same pattern where
they take encryption context as the first parameter, and the
boot_enc_decrypt stands out here as it does not work around
the encryption context, but is rather single-part decryption
function only used for decrypting of the image encryption
key.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/bootutil/src/encrypted.c b/boot/bootutil/src/encrypted.c
index 44975cc..d8e3292 100644
--- a/boot/bootutil/src/encrypted.c
+++ b/boot/bootutil/src/encrypted.c
@@ -415,7 +415,7 @@
  * @param enckey An AES-128 or AES-256 key sized buffer to store to plain key.
  */
 int
-boot_enc_decrypt(const uint8_t *buf, uint8_t *enckey)
+boot_decrypt_key(const uint8_t *buf, uint8_t *enckey)
 {
 #if defined(MCUBOOT_ENCRYPT_RSA)
     bootutil_rsa_context rsa;
@@ -660,7 +660,7 @@
         return -1;
     }
 
-    return boot_enc_decrypt(buf, bs->enckey[slot]);
+    return boot_decrypt_key(buf, bs->enckey[slot]);
 }
 
 bool