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/bootutil_misc.c b/boot/bootutil/src/bootutil_misc.c
index 614c2e0..4620213 100644
--- a/boot/bootutil/src/bootutil_misc.c
+++ b/boot/bootutil/src/bootutil_misc.c
@@ -283,7 +283,7 @@
         }
         /* Only try to decrypt non-erased TLV metadata */
         if (i != BOOT_ENC_TLV_ALIGN_SIZE) {
-            rc = boot_enc_decrypt(bs->enctlv[slot], bs->enckey[slot]);
+            rc = boot_decrypt_key(bs->enctlv[slot], bs->enckey[slot]);
         }
     }
 #else