Log Message:
guide: Document TOOL_DEPENDS

Document TOOL_DEPENDS and clarify the differencies with BUILD_DEPENDS.
Adjust previous BUILD_DEPENDS examples (all of them were actually
TOOL_DEPENDS).

XXX: No BUILD_DEPENDS example is provided, if you have any good and common
XXX: BUILD_DEPENDS example please add it (a TODO comment was added regarding
XXX: that).  At least according a quick grep of BUILD_DEPENDS probably most of
XXX: them are actually BUILD_DEPENDS and maybe all real BUILD_DEPENDS are via
XXX: buildlink3.mk inclusion.