Hi Yi, Wilson,
Using the LTIB
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So far we know that the LTIB is downloading source from http://www.bitshrine.org/ltib/ when it is building the rootfs image (Not from ubuntu.com) and the distribution package source is already there, somebody maintain it.
Using the default Ubuntu 10.04 Image
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When we run "sudo apt-get install [package]", naturally, it source the package from http://ports.ubuntu.com/ubuntu-ports/ so as Wilson already said the Ubuntu 10.04 (lucid) was indeed a pre-built image from Canonical is correct.
Linux is consist of Kernel, Library, Modules and Others file and for the boot process, we are using U-boot. We can separate the Kernel and the rest like Library, Modules and Others become what we call ROOTFS or IMAGE. What I can understand is that using LTIB, we are creating our own custom u-boot, kernel and rootfs which can run well with iMX53 QSB (since it is supported by Freescale) but it will be different from the default demo image which is Ubuntu 10.04 (Lucid). The default windows manager is already a big different. The build process of the default Ubuntu image I guess is not using LTIB anyway. But of course, in Linux world, all the source is out there. If we still want to use LTIB to build the Ubuntu image, we can perhaps somehow write some LTIB config file to fetch related source and all its dependencies sources from Canonical site, merged with the LTIB process flow then we can produce the ubuntu 10.04 or even any other version of Ubuntu. Of course, if someone already got this config file written then it will be fantastic !