[docs] README.md need to update for building on MacOS

[qiaojingx]

Description

In order to build on MacOS, README.md need to update according to http://docs.zephyrproject.org/getting_started/installation_mac.html.

Test Code

README.md

Actual Result

Build with BOARD=frdm_k64f will fail with error Kconfig not found by following instruction building-and-running-on-macos.

Expected Result

Build with BOARD=frdm_k64f succeed.

Test Builds

Branch	Commit Id	Target Device	Test Date	Result
master	1b970c8	MacOS	Dec 13, 2017	Fail

Additional Information

execute below commands before build, will fix Kconfig not found issue.

Build Kconfig in $ZEPHYR_BASE/build and add it to path

$ cd $ZEPHYR_BASE
$ mkdir build && cd build
$ cmake $ZEPHYR_BASE/scripts
$ make
$ echo "export PATH=$PWD/kconfig:\$PATH" >> $HOME/.zephyrrc
$ source $ZEPHYR_BASE/zephyr-env.sh

[jimmy-huang]

@qiaojingx I've created a patch #1780 to address some of these issues, can you see if anything else needs to be updated?

[qiaojingx]

Verified with commit 0c0af9b. README.md has been updated. Close.