The library/net.c and its corresponding include/mbedtls/net.h file are
renamed to library/net_sockets.c and include/mbedtls/net_sockets.h
respectively. This is to avoid naming collisions in projects which also
have files with the common name 'net'.
Ideally, Makefile targets should be called irrespectively of their
execution order. In this sense, `clean` and `lib` should not be
passed both as target command line arguments in the same make
invocation. This is because out-of-order execution could cause a
failure. This change ensures that `clean` and `lib` are called in
separate executions of make in scripts/footprint.sh
