configs: Rename PXE_SERIAL_CONSOLE to IPXE_SERIAL_CONSOLE

This follows commit 238ff1e9c which did the rename across the tree
except here. Since Kconfig is going to become more strict about unknown
symbols, fix it.

Change-Id: Ic31b8ae353ec07e8b8adab46b604365be4be44d9
Signed-off-by: Patrick Georgi <patrick@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79252
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/configs/config.pcengines_apu4 b/configs/config.pcengines_apu4
index 9e7ca5a..17db794 100644
--- a/configs/config.pcengines_apu4
+++ b/configs/config.pcengines_apu4
@@ -8,5 +8,5 @@
 CONFIG_PXE=y
 CONFIG_BUILD_IPXE=y
 CONFIG_PXE_ROM_ID="8086,1539"
-# CONFIG_PXE_SERIAL_CONSOLE is not set
+# CONFIG_IPXE_SERIAL_CONSOLE is not set
 CONFIG_MEMTEST_SECONDARY_PAYLOAD=y