When the default tune for x86_64 was changed to core2-64, the
environment setup script name did not contain x86 anymore. Hence, the
adt_installer failed for x86_64.
This commit contains a generic fix and is supposed to work with any kind
of machine/tune setting. It's actually extracting the environment script
name using 'opkg files meta-environment-MACHINE'. So, no need to do any
other sort of searches. We know exactly which is the environment setup
script for the specified machine.
