Fix _str_default() to return implicit zero for INT/HEX

sym_get_string_default() in scripts/kconfig/symbol.c (Linux) returns
"0" for INT and "0x0" for HEX when no explicit default exists.
Kconfiglib's _str_default() was returning "" instead, causing
write_min_config() to include hex symbols whose value matches the
implicit zero default (e.g. DEBUG_UART_PHYS=0x0 in arm rpc_defconfig).

Author: jserv

kconfiglib.py

@@ -5212,6 +5212,13 @@ def _str_default(self):
                 if expr_value(cond):
                     return default.str_value
 
+            # sym_get_string_default() in scripts/kconfig/symbol.c (Linux)
+            # returns "0" for INT and "0x0" for HEX when no default exists.
+            if self.orig_type is INT:
+                return "0"
+            if self.orig_type is HEX:
+                return "0x0"
+
         return ""
 
     def _warn_select_unsatisfied_deps(self):