util/kconfig: Fix default value getter for integer options

CB:37152 was supposed to be uprev to Linux's kconfig, but it got this
one case wrong, Linux never returned "0" [1]. As a result, when an
option has default value different than 0, and it was changed to 0,
savedefconfig skips saving it. However, during the build from such
defconfig the option is assigned default value.

TEST=Set SEABIOS_DEBUG_LEVEL to 0 and see that savedefconfig writes
it to defconfig file.

[1] https://github.com/torvalds/linux/commit/7cf3d73b4360e91b14326632ab1aeda4cb26308d

Signed-off-by: Krystian Hebel <krystian.hebel@3mdeb.com>
Change-Id: I821e45dcec99904fab85f136298cbd0315237ff6
Reviewed-on: https://review.coreboot.org/c/coreboot/+/72650
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michał Żygowski <michal.zygowski@3mdeb.com>
diff --git a/util/kconfig/symbol.c b/util/kconfig/symbol.c
index 7c687b0..6cbbac4 100644
--- a/util/kconfig/symbol.c
+++ b/util/kconfig/symbol.c
@@ -757,7 +757,7 @@
 		}
 	case S_INT:
 	case S_HEX:
-		return "0";
+		return str;
 	case S_STRING:
 		return str;
 	case S_UNKNOWN: