build: improve error message for missing dependency

build: improve error message for missing dependency

When building a driver or library and a required internal dependency
is missing, the error message printed was:

    Tried to get unknown variable "shared_rte_<libname>".

However, a better way to handle this is to catch the missing variable
earlier and convert it into a proper message, informing the user
that the required dependency is missing, and most importantly, what
library or driver wanted that dependency.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>