Ubuntu cross compile for RPi for buildroot

on your build system

cd rtl8812au
export CROSS_COMPILE=$(HOME)/buildroot/output/host/bin/arm-buildroot-linux-uclibcgnueabihf-
export KSRC=$(HOME)/buildroot/output/build/linux-custom
export LD_LIBRARY_PATH=$(HOME)/buildroot/output/host/lib

edit the Makefile

CONFIG_PLATFORM_I386_PC = n  <- change this from y to n

add below line to Makefile somewhere near other CONFIG_PLATFORM entries

CONFIG_PLATFORM_ARM_RPI_CROSS = y

add below lines to Makefile somewhere near ifeq ($(CONFIG_PLATFORM_ARM_RPI),y)

ifeq ($(CONFIG_PLATFORM_ARM_RPI_CROSS), y)
EXTRA_CFLAGS += -DCONFIG_LITTLE_ENDIAN
ARCH := arm
CROSS_COMPILE ?= arm-buildroot-linux-uclibcgnueabihf-
KVER ?= $(shell uname -r)
KSRC ?= /lib/modules/$(KVER)/build
endif

Build
make