[slirp] Add libslirp driver for Linux

Add a driver using libslirp to provide a virtual network interface
without requiring root permissions on the host.  This simplifies the
process of running iPXE as a Linux userspace application with network
access.  For example:

  make bin-x86_64-linux/slirp.linux
  ./bin-x86_64-linux/slirp.linux --net slirp

libslirp will provide a built-in emulated DHCP server and NAT router.
Settings such as the boot filename may be controlled via command-line
options.  For example:

  ./bin-x86_64-linux/slirp.linux \
      --net slirp,filename=http://192.168.0.1/boot.ipxe

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.linux

@@ -26,9 +26,21 @@ NON_AUTO_MEDIA = linux
 #
 LINUX_CFLAGS	+= -Os -idirafter include -DSYMBOL_PREFIX=$(SYMBOL_PREFIX)
 
+# Check for libslirp
+#
+LIBSLIRP_TEST = $(CC) $(LINUX_CFLAGS) -x c /dev/null -nostartfiles \
+		      -include slirp/libslirp.h -lslirp \
+		      -o /dev/null >/dev/null 2>&1
+WITH_LIBSLIRP	:= $(shell $(LIBSLIRP_TEST) && $(ECHO) yes)
+ifneq ($(WITH_LIBSLIRP),)
+LINUX_CFLAGS	+= -DHAVE_LIBSLIRP
+LINUX_LDFLAGS	+= -lslirp
+endif
+
 # Host API wrapper
 #
-$(BIN)/linux_api.o : interface/linux/linux_api.c $(MAKEDEPS)
+$(BIN)/linux_api.o : interface/linux/linux_api.c include/ipxe/linux_api.h \
+		     include/ipxe/slirp.h $(MAKEDEPS)
 	$(QM)$(ECHO) "  [BUILD] $@"
 	$(Q)$(CC) $(LINUX_CFLAGS) $(WORKAROUND_CFLAGS) -o $@ -c $<
 
@@ -36,4 +48,5 @@ $(BIN)/linux_api.o : interface/linux/linux_api.c $(MAKEDEPS)
 #
 $(BIN)/%.linux : $(BIN)/%.linux.tmp $(BIN)/linux_api.o
 	$(QM)$(ECHO) "  [FINISH] $@"
-	$(Q)$(CC) $(LINUX_CFLAGS) $(WORKAROUND_CFLAGS) -o $@ $^
+	$(Q)$(CC) $(LINUX_CFLAGS) $(WORKAROUND_CFLAGS) $(LINUX_LDFLAGS) \
+		-o $@ $^