配置linux内核支持pppd

linux内核自带支持pppd，所以我们在这里配置内核就可以了。
make menuconfig

    Device Drivers->

     ->Network Device Support->

       <*> PPP (point-to-point protocol) support

       [*]   PPP multilink support

       <*> PPP support for async serial ports

       <*> PPP support for sync tty ports

       <*> SLIP (serial line) support

       [*]   CSLIP compressed headers