This is to revert the changes for recursively calling build-baremetal-toolchain.sh

We decided to revert these changes incase of unexpected behaviour
and have instructed users how to use the build-newlib-for-mingw-toolchain.sh
script in the Readme.
We have also renamed build-newlib.sh to build-newlib-for-mingw-toolchain.sh
I have also done some slight alterations in this script.

More info - GNUTOOLS-15453

Change-Id: I7af7cdfe54f81962662e5c19e7ed303f60b89619