[nrf fromtree] sysbuild: Fix Kconfig symbol SIGNATURE_TYPE

Add missing quotes around string values, so that they won't be
interpreted as undefined symbol names.

Signed-off-by: Grzegorz Swiderski <[email protected]>
(cherry picked from commit 17ba479)

Author: 57300

share/sysbuild/images/bootloader/Kconfig

@@ -116,10 +116,10 @@ endchoice
 
 config SIGNATURE_TYPE
 	string
-	default NONE if BOOT_SIGNATURE_TYPE_NONE
-	default RSA  if BOOT_SIGNATURE_TYPE_RSA
-	default ECDSA_P256 if BOOT_SIGNATURE_TYPE_ECDSA_P256
-	default ED25519 if BOOT_SIGNATURE_TYPE_ED25519
+	default "NONE" if BOOT_SIGNATURE_TYPE_NONE
+	default "RSA" if BOOT_SIGNATURE_TYPE_RSA
+	default "ECDSA_P256" if BOOT_SIGNATURE_TYPE_ECDSA_P256
+	default "ED25519" if BOOT_SIGNATURE_TYPE_ED25519
 
 choice BOOT_SIGNATURE_TYPE
 	prompt "Signature type"