BUILD: makefile: add a new generic target "tiny"

BUILD: makefile: add a new generic target "tiny"

This target disables all possible features except poll(). It is meant to
serve as a base for small embedded setups, on top of which one may manually
enable select features. Even threads, traces/h2/fcgi/SPOE are disabled.
The default executable is roughly 80% smaller than with linux-glibc:

  $ size haproxy-linux-glibc haproxy-tiny
     text    data     bss      dec    hex filename
  3660924  176964 9868784 13706672 d125b0 haproxy-linux-glibc
  2537864  146512   84928  2769304 2a4198 haproxy-tiny

With SSL enabled, the difference shrinks a bit (-77%):

  $ size haproxy-linux-glibc-ssl haproxy-tiny-ssl
     text    data     bss      dec    hex filename
  4163373  208788 9873904 14246065 d960b1 haproxy-linux-glibc-ssl
  2950852  177732   90048  3218632 311cc8 haproxy-tiny-ssl