Update README.md to recommend passing CROSS_COMPILE via env

Building u-boot should pass CROSS_COMPILE via the environment
rather than a make variable; some ARM64 boards require parts of
the bootloader to be built w/ 32-bit toolchains, setting
CROSS_COMPILE via a make variable prevents that override from
working.

Change-Id: Ia277fd44f6a115103fed82943556c01f5c961f3e

README.md

diff --git a/README.md b/README.md
index c3cb122..1779f85 100644
--- a/README.md
+++ b/README.md
@@ -34,7 +34,7 @@
 ```
 3. Build u-boot.bin
 ```
-make CROSS_COMPILE=aarch64-linux-gnu-
+CROSS_COMPILE=aarch64-linux-gnu- make
 ```
 
 You now have a u-boot.bin that you can flash to a target with using fastboot.