We need to define a lot of terms and nouns that will be used often into this document:
embedded system: It's a complete computer system reduced into a single (ofter very) small board. This kind of systems are designed to work in critical environments such as industrial or aerospace. They're normally equiped with flash memory, disk on module, or disk on chip, as the main mass storage memory. Embedded systems are generally piloted by ad-hoc designed operating systems able to use in the better way the system resources.
host system: This is the Gentoo system that we're going to use for building the embedded operating system. The better is the power of this host (in terms of cpu, ram and disks), the fewer amount of time we will need to compile packages.
system_rootfs: this is the root filesystem of our host system.
development_rootfs: this is the root file system used to compile the root filesystem of our embedded os.
embedded_rootfs: the root filesystem of our embedded os. The one that, at the end of the process, must be copied into the embedded's mass storage disk.