Create ARM environment on Debian/Ubuntu using Qemu

[herry13]

Currently, the building process was done on a RaspberryPi which is sometime quite slow. Moving the build to Qemu environment on a more powerful x86 machine might speed up the process.

This might also help if we want to use public CI such as Travis-CI.

[herry13]

The existing kernel for Qemu uses versatile mode which has maximum RAM of 256MB -- not big enough to compile large program. Need at least 2GB, or if possible 4GB.