configure: Add --disable-firmware option to avoid building firmware

Allow usbboot to be built on systems without a mipsel cross-compiler
by passing configure the --disable-firmware option.

Before using the resulting binary, one would need to grab the firmware
from another machine.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

usbboot/src/Makefile.am

@@ -12,8 +12,11 @@ usbboot_SOURCES = cmd.c  command_line.c  ingenic_cfg.c  \
 	ingenic_usb.c  main.c
 
 pkgdatadir = $(datadir)/xburst-tools/
+
+if COND_FIRMWARE
 pkgdata_DATA = ../xburst_stage1/xburst_stage1.bin \
 	../xburst_stage2/xburst_stage2.bin
+endif
 
 cfgdir = $(sysconfdir)/xburst-tools/
 cfg_DATA = ../doc/usbboot.cfg