Add PKCS#1 1.15 config option

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/mynewt/syscfg.yml b/boot/mynewt/syscfg.yml
index afeaabe..a1e23cb 100644
--- a/boot/mynewt/syscfg.yml
+++ b/boot/mynewt/syscfg.yml
@@ -31,6 +31,9 @@
     BOOTUTIL_SIGN_RSA:
         description: 'Images are signed using RSA2048.'
         value: 0
+    BOOTUTIL_RSA_PKCS1_15:
+        description: 'Sign using old PKCS#1 1.5 (otherwise uses PSS)'
+        value: 0
     BOOTUTIL_SIGN_EC:
         description: 'Images are signed using ECDSA NIST P-224.'
         value: 0