Set the default prefix to /opt/acg-toolchain

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>
Add support for building out-of-tree. Closes #4

cd ${HOME}
git clone https://github.com/bhundven/autocrossgen
cd autocrossgen
./autogen.sh
cd ${HOME}
mkdir arm-acg-build
cd arm-acg-build
../autocrossgen/configure --prefix=${HOME}/arm-acg-toolchain --target=arm-unknown-linux-gnueabi
make all

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>