Update ring+gcc crates and declare sig-ecdsa feature

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/sim/mcuboot-sys/Cargo.toml b/sim/mcuboot-sys/Cargo.toml
index 31f4504..220972b 100644
--- a/sim/mcuboot-sys/Cargo.toml
+++ b/sim/mcuboot-sys/Cargo.toml
@@ -14,14 +14,14 @@
 # compile with both sig-rsa and sig-ecdsa enabled.
 sig-rsa = []
 
-# Verify ECDSA signatures.
+# Verify ECDSA (secp256r1) signatures.
 sig-ecdsa = []
 
 # Overwrite only upgrade
 overwrite-only = []
 
 [build-dependencies]
-gcc = "0.3.51"
+gcc = "0.3.54"
 
 [dependencies]
 lazy_static = "0.2"