[readme] Update cmake instructions to match Zephyr upstream (#1718)

The ZJS cmake instructions were wrong (and did not seem to work)
for ZJS. Updated the the same steps that zephyr uses. These
steps are also much easier and do not require actually building
cmake.

Signed-off-by: James Prestwood <[email protected]>

Author: James Prestwood

README.md

@@ -62,19 +62,6 @@ sudo apt-get update
 sudo apt-get install cmake dfu-util git python3-yaml screen uglifyjs
 ```
 
-Note: cmake 3.8.2 or later is required, and your system might install an older
-version, if this is the case, you'll have to manually install the latest
-version of cmake:
-```bash
-sudo apt-get purge cmake
-wget https://cmake.org/files/v3.9/cmake-3.9.5.tar.gz
-tar -xzvf cmake-3.9.5.tar.gz
-cd cmake-3.9.5/
-./bootstrap
-make -j4
-sudo make install
-```
-
 Note: python3-yaml is a recent requirement for the frdm-k64f build due to a
 change in Zephyr, so it could be left out currently if you don't use k64f.
 Before that, for a while python-yaml was needed when the script was using
@@ -180,6 +167,18 @@ environment variables, too. Here's the right way to do that:
 source deps/zephyr/zephyr-env.sh
 ```
 
+Note: cmake 3.8.2 or later is required, and your system might install an older
+version, if this is the case, you'll have to manually install the latest
+version of cmake:
+```bash
+$ mkdir $HOME/cmake && cd $HOME/cmake
+$ wget https://cmake.org/files/v3.8/cmake-3.8.2-Linux-x86_64.sh
+$ yes | sh cmake-3.8.2-Linux-x86_64.sh | cat
+$ echo "export PATH=$PWD/cmake-3.8.2-Linux-x86_64/bin:\$PATH" >> $HOME/.zephyrrc
+$ source $ZJS_ROOT/deps/zephyr/zephyr-env.sh
+$ cmake --version
+```
+
 ### Build and Flash
 #### x86 application image and ARC support image
 Now you're ready to build the x86 and ARC images. The x86 image