BUILD: makefile: clarify the "help" output and list options

BUILD: makefile: clarify the "help" output and list options

The list of enable and disabled build options now appears separately
at the end of "make help". This is convenient to know what is enabled
by default on a given target. For example :

  $ make help TARGET=linux2628
  Enabled features for TARGET 'linux2628' (disable with 'USE_xxx=') :
    EPOLL NETFILTER POLL THREAD TPROXY LINUX_TPROXY LINUX_SPLICE LIBCRYPT
    CRYPT_H FUTEX ACCEPT4 CPU_AFFINITY DL RT PRCTL THREAD_DUMP

  Disabled features for TARGET 'linux2628' (enable with 'USE_xxx=1') :
    KQUEUE MY_EPOLL MY_SPLICE PCRE PCRE_JIT PCRE2 PCRE2_JIT PRIVATE_CACHE
    PTHREAD_PSHARED REGPARM STATIC_PCRE STATIC_PCRE2 VSYSCALL GETADDRINFO
    OPENSSL LUA MY_ACCEPT4 ZLIB SLZ TFO NS DEVICEATLAS 51DEGREES WURFL
    SYSTEMD OBSOLETE_LINKER EVPORTS