soc/amd/{cezanne,picasso}: Escape PSP_VERSTAGE_FILE default

If we don't escape the $ then the actual $(obj) path will be written
into the .config file. With this change `$(obj)` is written into the
.config file. The Makefile then does:

    PSP_VERSTAGE_FILE=$(call strip_quotes,$(CONFIG_PSP_VERSTAGE_FILE))

Since this is a recursive assignment the $(obj) will be expanded at that
point.

This change makes it easier to compare full .config files.

BUG=none
TEST=Build ezkinil

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: Ic961df148d3f22585f3441d75c3f2454329c678a
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56395
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kangheui Won <khwon@chromium.org>
diff --git a/src/soc/amd/picasso/Kconfig b/src/soc/amd/picasso/Kconfig
index 88bff50..eb1780a 100644
--- a/src/soc/amd/picasso/Kconfig
+++ b/src/soc/amd/picasso/Kconfig
@@ -432,7 +432,7 @@
 config PSP_VERSTAGE_FILE
 	string "Specify the PSP_verstage file path"
 	depends on VBOOT_STARTS_BEFORE_BOOTBLOCK
-	default "$(obj)/psp_verstage.bin"
+	default "\$(obj)/psp_verstage.bin"
 	help
 	  Add psp_verstage file to the build & PSP Directory Table