cross-compilation: use $host instead of $target

$target should be used only when building a compiler. It indicates
the target system for which the compiler will build executables.

$host is the variable that should be used to indicate the type of
system the package we are compiling now will run on.

Also, update the README to say to use --host instead of --target.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>