board: amlogic: vim: Fix README

Unlike the BayLibre repos Khadas' repo defaults to using native gcc,
so specify the CROSS_COMPILE setting matching the prescribed toolchain.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
diff --git a/board/amlogic/p212/README.khadas-vim b/board/amlogic/p212/README.khadas-vim
index b194236..a2c7606 100644
--- a/board/amlogic/p212/README.khadas-vim
+++ b/board/amlogic/p212/README.khadas-vim
@@ -50,7 +50,7 @@
  > git clone https://github.com/khadas/u-boot -b Vim vim-u-boot
  > cd vim-u-boot
  > make kvim_defconfig
- > make
+ > make CROSS_COMPILE=aarch64-none-elf-
  > export FIPDIR=$PWD/fip
 
 Go back to mainline U-Boot source tree then :