创建，添加sdram

rt-thread/components/drivers/usb/usbhost/SConscript

@@ -0,0 +1,34 @@
+Import('RTT_ROOT')
+from building import *
+
+cwd = GetCurrentDir()
+src = Split("""
+core/usbhost_core.c
+core/driver.c
+core/usbhost.c
+core/hub.c
+""")
+
+if GetDepend('RT_USBH_ADK'):
+    src += Glob('class/adk.c')
+    src += Glob('class/adkapp.c')
+
+if GetDepend('RT_USBH_MSTORAGE'):
+    src += Glob('class/mass.c')
+    src += Glob('class/udisk.c')
+
+if GetDepend('RT_USBH_HID'):
+    src += Glob('class/hid.c')
+
+if GetDepend('RT_USBH_HID_MOUSE'):
+    src += Glob('class/umouse.c')
+
+if GetDepend('RT_USBH_HID_KEYBOARD'):
+    src += Glob('class/ukbd.c')
+
+CPPPATH = [cwd, cwd + '/class', cwd + '/core', \
+    cwd + '/include', cwd + '../../../include']
+
+group = DefineGroup('rt_usbh', src, depend = ['RT_USING_USB_HOST'], CPPPATH = CPPPATH)
+
+Return('group')