[doc] Document use flags

[imphil]

FuseSoC supports use flags, but doesn't document which use flags are actually set. This needs to be documented.

Currently we set:

• A target use flag target_TARGETNAME, e.g. target_sim if fusesoc is called with --target=sim.
• A tool use flags tool_TOOLNAME, e.g. tool_verilator if fusesoc is called with --tool=verilator

Support for user-defined use flags is being developed in #268, but currently not present.

[olofk]

A rudimentary first version of user-defined use flags are actually implemented. It's possible to specify --flag=i_am_a_flag --flag=i_am_another_flag on the command line