Sorry for the misunderstanding! We can install CentOS into USB and boot from USB, it works fine like that.
And we did not say that it can be installed into internal eMMC, because CentOS does not include the eMMC drivers while other distributions support eMMC well.
Actually, if you know how to build kernel, you can rebuild kernel with eMMC driver on CentOS 6.5 and later version, then DD the whole system into eMMC flash, it will boot from eMMC and works fine; If you are not able to do that, pls use other distributions.