arm: mediatek: merge board Kconfigs into mach-mediatek

On MediaTek boards we cannot override the SYS_BOARD / SYS_CONFIG_NAME
variables from defconfig.
This is because in board/mediatek/mtXXXX/Kconfig this value was override
by default due to the if CONFIG_TARGET_MTXXXX condition.

Merge all the Kconfigs to the mach-medatek/Kconfig.

This way:
- we only define SYS_{SOC,VENDOR} once
- all board definitions are in a single place, simplifying the build logic.

Signed-off-by: Guillaume La Roque <glaroque@baylibre.com>
diff --git a/arch/mips/mach-mtmips/mt7620/Kconfig b/arch/mips/mach-mtmips/mt7620/Kconfig
index 5db83eb..3ca711a 100644
--- a/arch/mips/mach-mtmips/mt7620/Kconfig
+++ b/arch/mips/mach-mtmips/mt7620/Kconfig
@@ -66,6 +66,12 @@
 	default 6 if CPU_FREQ_600MHZ
 	default 7 if CPU_FREQ_620MHZ
 
-source "board/mediatek/mt7620/Kconfig"
+config SYS_CONFIG_NAME
+	string "Board configuration name"
+	default "mt7620" if  BOARD_MT7620_RFB || BOARD_MT7620_MT7530_RFB
+
+config SYS_BOARD
+	string "Board name"
+	default "mt7620" if  BOARD_MT7620_RFB || BOARD_MT7620_MT7530_RFB
 
 endif