[Makefile] Remove mbedtls's config.h on first build in lib/Makefile
Previous commit fixed the dependency on crypto/config.h (it must be
copied into mbedtls include directory). However, config.h with
default and incorrect settings already exists in mbedtls include
dir after its download, and Graphene needs to replace it with our
version of configuration. This commit simply removes default config.h
on first build of mbedTLS to force the dependency introduced in
the previous commit.
Dmitrii Kuvaiskii