zephyr: Support RSA, and ECDSA P-256 signing

Make it clear in the top-level Makefile how to configure mcuboot for
Zephyr for a particular signing algorithm.  Currently supported, are the
RSA signatures, and ECDSA with the P-256 curve.  These configuration
lines will select the code built in the bootloader, as well as which
public key gets included with the image.

This also adds a demo public key for the P-256 signatures.
diff --git a/boot/zephyr/keys.c b/boot/zephyr/keys.c
index 051eb8a..9a06f3c 100644
--- a/boot/zephyr/keys.c
+++ b/boot/zephyr/keys.c
@@ -19,6 +19,7 @@
 
 #include <bootutil/sign_key.h>
 
+#if defined(BOOTUTIL_SIGN_RSA)
 const unsigned char root_pub_der[] = {
   0x30, 0x82, 0x01, 0x0a, 0x02, 0x82, 0x01, 0x01, 0x00, 0xd1, 0x06, 0x08,
   0x1a, 0x18, 0x44, 0x2c, 0x18, 0xe8, 0xfb, 0xfd, 0xf7, 0x0d, 0xa3, 0x4f,
@@ -45,6 +46,24 @@
   0xc9, 0x02, 0x03, 0x01, 0x00, 0x01
 };
 const unsigned int root_pub_der_len = 270;
+#elif defined(BOOTUTIL_SIGN_EC256)
+const unsigned char root_pub_der[] = {
+	0x30, 0x59, 0x30, 0x13, 0x06, 0x07, 0x2a, 0x86,
+	0x48, 0xce, 0x3d, 0x02, 0x01, 0x06, 0x08, 0x2a,
+	0x86, 0x48, 0xce, 0x3d, 0x03, 0x01, 0x07, 0x03,
+	0x42, 0x00, 0x04, 0x2a, 0xcb, 0x40, 0x3c, 0xe8,
+	0xfe, 0xed, 0x5b, 0xa4, 0x49, 0x95, 0xa1, 0xa9,
+	0x1d, 0xae, 0xe8, 0xdb, 0xbe, 0x19, 0x37, 0xcd,
+	0x14, 0xfb, 0x2f, 0x24, 0x57, 0x37, 0xe5, 0x95,
+	0x39, 0x88, 0xd9, 0x94, 0xb9, 0xd6, 0x5a, 0xeb,
+	0xd7, 0xcd, 0xd5, 0x30, 0x8a, 0xd6, 0xfe, 0x48,
+	0xb2, 0x4a, 0x6a, 0x81, 0x0e, 0xe5, 0xf0, 0x7d,
+	0x8b, 0x68, 0x34, 0xcc, 0x3a, 0x6a, 0xfc, 0x53,
+	0x8e, 0xfa, 0xc1, };
+const unsigned int root_pub_der_len = 91;
+#else
+#error "No public key available for given signing algorithm."
+#endif
 
 const struct bootutil_key bootutil_keys[] = {
 	{