Install buildhost's toolchain inside buildroot when cross-compiling

* build-epiphany-sdk.sh: If we are cross compiling, instruct
'build-toolchain.sh' to install the build-host toolchain inside
the 'builds' directory. Do not install to '/opt/adapteva'. This
would break the build when the user did not have write permissions.
Also, ensure that the toolchain is in PATH so 'install-sdk.sh'
will find it.
* build-toolchain.sh: Remove an erroneous and effectively no-op PATH
statement.
* install-sdk.sh: Only guess Epiphany toolchain's path when it is
not already in the path.
(check_toolchain): Helper function. Checks if a specific toolchain is
available.
Epiphany SDK 2014.11-rc2