mb/*/Kconfig: Annotate closing endif with corresponding condition

It’s common to annotate the closing endif, so do it for these four
files.

Change-Id: Ia5d071e1f544c9dea5af9c6bc3c605d9a0c5c0f5
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47477
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

src/mainboard/clevo/Kconfig

@@ -12,4 +12,4 @@ source "src/mainboard/clevo/*/Kconfig"
 config MAINBOARD_VENDOR
 	default "Clevo"
 
-endif
+endif # VENDOR_CLEVO

src/mainboard/razer/Kconfig

@@ -12,4 +12,4 @@ source "src/mainboard/razer/*/Kconfig"
 config MAINBOARD_VENDOR
 	default "RAZER"
 
-endif
+endif # VENDOR_RAZER

src/mainboard/system76/Kconfig

@@ -12,4 +12,4 @@ source "src/mainboard/system76/*/Kconfig"
 config MAINBOARD_VENDOR
 	default "System76"
 
-endif
+endif # VENDOR_SYSTEM76

src/mainboard/up/Kconfig

@@ -12,4 +12,4 @@ source "src/mainboard/up/*/Kconfig"
 config MAINBOARD_VENDOR
 	default "UP"
 
-endif
+endif # VENDOR_UP