Create a generic a macro to install packages
Embtoolkit build system needs a xxx package to have a xxx_install
makefile target. The build system, when the package is selected
to be in the generated root filesystem, calls xxx_install target.
The xxx_install target is the same in almost all targets which use
an autotools configure script, so it is worth to have a generic
EMBTK_INSTALL_PKG makefile macro.