kconfig: silence warning when parsing auto.conf when a symbol has changed type

When a symbol changes type from tristate to bool, and was previously set to
'm', a subsequent silentoldconfig would warn about inconsistency, such as:

    include/config/auto.conf:3014:warning: symbol value 'm' invalid for
    HOTPLUG_PCI_PCIE

Seen by Linus with the merge in aa8032b6 (sequence to reproduce by Michal):
    git checkout 1fe0135b
    make mrproper
    make allmodconfig
    make silentoldconfig
    git checkout aa8032b6
    make allmodconfig
    make silentoldconfig

Since HOTPLUG_PCI_PCIE changed from tristate to bool in aa8032b6, it was
previously set to 'm' in auto.conf by the first allmodconfig+silentoldconfig,
but then was set to 'y' by the second allmodconfig. Then the second
silentoldconfig prints the warning.

The warning in this case is a spurious warning, which happens at the time
kconfig tries to detect symbols that have changed, to touch the empty
header files in include/config used for dependency-tracking by make.

Silence the warning when we read the old auto.conf file, since it is
perfectly legit that a symbol changed type since the previous call.

Thread in:
    http://marc.info/?l=linux-pci&m=137569198904000&w=2Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>

scripts/kconfig/confdata.c

@@ -140,7 +140,9 @@ static int conf_set_sym_val(struct symbol *sym, int def, int def_flags, char *p)
 			sym->flags |= def_flags;
 			break;
 		}
-		conf_warning("symbol value '%s' invalid for %s", p, sym->name);
+		if (def != S_DEF_AUTO)
+			conf_warning("symbol value '%s' invalid for %s",
+				     p, sym->name);
 		return 1;
 	case S_OTHER:
 		if (*p != '"') {
@@ -161,6 +163,7 @@ static int conf_set_sym_val(struct symbol *sym, int def, int def_flags, char *p)
 			memmove(p2, p2 + 1, strlen(p2));
 		}
 		if (!p2) {
+			if (def != S_DEF_AUTO)
 				conf_warning("invalid string found");
 			return 1;
 		}
@@ -172,7 +175,9 @@ static int conf_set_sym_val(struct symbol *sym, int def, int def_flags, char *p)
 			sym->def[def].val = strdup(p);
 			sym->flags |= def_flags;
 		} else {
-			conf_warning("symbol value '%s' invalid for %s", p, sym->name);
+			if (def != S_DEF_AUTO)
+				conf_warning("symbol value '%s' invalid for %s",
+					     p, sym->name);
 			return 1;
 		}
 		break;