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/Kconfig b/arch/mips/mach-mtmips/Kconfig
index 9f300a9..151b004 100644
--- a/arch/mips/mach-mtmips/Kconfig
+++ b/arch/mips/mach-mtmips/Kconfig
@@ -1,6 +1,9 @@
 menu "MediaTek MIPS platforms"
 	depends on ARCH_MTMIPS
 
+config SYS_VENDOR
+	default "mediatek" if BOARD_MT7628_RFB || BOARD_MT7620_RFB || BOARD_MT7620_MT7530_RFB
+
 config SYS_MALLOC_F_LEN
 	default 0x1000