Add simulator support for RSA-3072 sigs

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/caps.c b/boot/bootutil/src/caps.c
index b449a6b..f2126d7 100644
--- a/boot/bootutil/src/caps.c
+++ b/boot/bootutil/src/caps.c
@@ -22,8 +22,13 @@
     uint32_t res = 0;
 
 #if defined(MCUBOOT_SIGN_RSA)
+#if MCUBOOT_SIGN_RSA_LEN == 2048
     res |= BOOTUTIL_CAP_RSA2048;
 #endif
+#if MCUBOOT_SIGN_RSA_LEN == 3072
+    res |= BOOTUTIL_CAP_RSA3072;
+#endif
+#endif
 #if defined(MCUBOOT_SIGN_EC)
     res |= BOOTUTIL_CAP_ECDSA_P224;
 #endif