util/kconfig: Add comment explaining difference from upstream

coreboot adds a patch on top of upstream Kconfig which allows the
generated Kconfig dependency files to be placed in a separate
directory than the autoconfig files based on the KCONFIG_SPLITCONFIG
variable. Add a comment to explain this difference.

Change-Id: Ief38ab84f852ff24f896ec8bbf094aa737a172d9
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69952
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Fred Reitberger <reitbergerfred@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

util/kconfig/confdata.c

@@ -1035,6 +1035,11 @@ static int conf_touch_deps(void)
 	struct symbol *sym;
 	int res, i;
 
+	/*
+	 * Upstream Kconfig sets depfile_path based on the directory
+	 * prefix of the autoconfig path, but coreboot overrides this
+	 * using the KCONFIG_SPLITCONFIG environment variable
+	 */
 	strcpy(depfile_path, conf_get_autobase_name());
 	depfile_prefix_len = strlen(depfile_path);