kconfig: zephyr: provide logic for setting key file, simplify prj.conf

Automate process of selecting correct .pem key file.

Zephyr users are familiar with using 'menuconfig' and similar tools
for seeing what options are available, so remove =n choices from
prj.conf which were used to show the available options.

Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index 0cf133a..a52c7d6 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -126,6 +126,10 @@
 
 config BOOT_SIGNATURE_KEY_FILE
 	string "PEM key file"
+	default "root-ec-p256.pem" if BOOT_SIGNATURE_TYPE_ECDSA_P256
+	default "root-ed25519.pem" if BOOT_SIGNATURE_TYPE_ED25519
+	default "root-rsa-3072.pem" if BOOT_SIGNATURE_TYPE_RSA && BOOT_SIGNATURE_TYPE_RSA_LEN=3072
+	default "root-rsa-2048.pem" if BOOT_SIGNATURE_TYPE_RSA && BOOT_SIGNATURE_TYPE_RSA_LEN=2048
 	default ""
 	help
 	  You can use either absolute or relative path.