src/mainboard: Escape variable expansion in Kconfig

Kconfig 5.8 interprets $(...) itself using environment variables, which
generally means that they expand to the empty string. \$(...) works
with both our current and new Kconfig with the desired behavior
(to pass it through unmodified).

Change-Id: I726567eeb61d2035560152677d2b4548c1472be9
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44584
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/dedede/Kconfig

@@ -54,8 +54,8 @@ config DRIVER_TPM_SPI_BUS
 
 config FMDFILE
 	string
-	default "src/mainboard/$(CONFIG_MAINBOARD_DIR)/chromeos-dedede-16MiB.fmd" if BOARD_ROMSIZE_KB_16384
-	default "src/mainboard/$(CONFIG_MAINBOARD_DIR)/chromeos-dedede-32MiB.fmd" if BOARD_ROMSIZE_KB_32768
+	default "src/mainboard/\$(CONFIG_MAINBOARD_DIR)/chromeos-dedede-16MiB.fmd" if BOARD_ROMSIZE_KB_16384
+	default "src/mainboard/\$(CONFIG_MAINBOARD_DIR)/chromeos-dedede-32MiB.fmd" if BOARD_ROMSIZE_KB_32768
 
 config MAINBOARD_DIR
 	string

src/mainboard/kontron/bsl6/Kconfig

@@ -40,7 +40,7 @@ config VARIANT_DIR
 
 config OVERRIDE_DEVICETREE
 	string
-	default "variants/$(CONFIG_VARIANT_DIR)/overridetree.cb"
+	default "variants/\$(CONFIG_VARIANT_DIR)/overridetree.cb"
 
 config CBFS_SIZE
 	hex

src/mainboard/ocp/deltalake/Kconfig

@@ -40,7 +40,7 @@ config MAX_SOCKET
 
 config FMDFILE
 	string
-	default "src/mainboard/$(CONFIG_MAINBOARD_DIR)/board.fmd"
+	default "src/mainboard/\$(CONFIG_MAINBOARD_DIR)/board.fmd"
 
 # Deltalake is single socket server, the platform design has 1 DIMM per channel
 config DIMM_MAX