prebuilt-common.sh: Distinguish host and target builds
David 'Digit' Turner authored
This patch modifies the helper functions inside prebuilt-common.sh
to distinguish between host and target builds more easily.

A target build is used to build target Android binaries, while a host
one is used to build binaries that will run on the host machine, i.e.
invoked by the NDK build script.

We will use host builds to generate the Windows helper binaries that
the NDK depends on (sed, awk, make, etc...) in future patches.

Change-Id: Ifbc2a112ee08fa42c9987ee26fee4539476e91a4
3da53797