板子启动ubuntu无法进入命令行，并打印如下log：
25 [ 6.921205] EXT4-fs (mmcblk0p2): couldn't mount as ext3 due to feature incompatibilities
26 [ 6.931860] EXT4-fs (mmcblk0p2): couldn't mount as ext2 due to feature incompatibilities
27 [ 7.608656] EXT4-fs (mmcblk0p2): mounted filesystem with ordered data mode. Opts: (null)
28 [ 7.616966] VFS: Mounted root (ext4 filesystem) on device 179:2.
29 [ 7.637198] devtmpfs: mounted
30 [ 7.640977] Freeing unused kernel memory: 512K (c0a01000 - c0a81000)
31 mount: mount point /dev/pts does not exist
32 mountall: mount /dev/pts [130] terminated with status 32
33 mountall: Filesystem could not be mounted: /dev/pts
34 mount: mount point /dev/shm does not exist
35 mountall: mount /dev/shm [131] terminated with status 32
36 mountall: Filesystem could not be mounted: /dev/shm
37 mount: mount point /dev/pts does not exist
38 mountall: mount /dev/pts [135] terminated with status 32
39 mountall: Filesystem could not be mounted: /dev/pts
40 mount: mount point /dev/shm does not exist
41 mountall: mount /dev/shm [136] terminated with status 32
42 mountall: Filesystem could not be mounted: /dev/shm
43
解决办法：在make menuconfig里去掉选项：Device Drivers ---> Generic Driver Options ---> Maintain a devtmpfs filesystem to mount at /dev