sm6250-common: define board platform rule

Taken from
https://source.codeaurora.org/quic/la/platform/vendor/qcom/atoll/tree/BoardConfig.mk?h=LA.UM.8.9.r1-09500-SM6xx.0#n6

Can be cross-checked with the following prop
ro.board.platform

BoardConfigCommon.mk

@@ -18,3 +18,6 @@ TARGET_2ND_ARCH_VARIANT := armv7-a-neon
 TARGET_2ND_CPU_ABI := armeabi-v7a
 TARGET_2ND_CPU_ABI2 := armeabi
 TARGET_2ND_CPU_VARIANT := cortex-a9
+
+# Platform
+TARGET_BOARD_PLATFORM := atoll