FOE write and read for different files

[qiayuanl]

Notice that FOE_fread use foe_cfg->fbuffer as
the reading data address, I can set the fbuffer to the address I want to read in foe_cfg_t, and I can use the FoE reading for only one file.

However, if I want to read(and write) multiple files, the fbuffer is different, i.e. maybe another write buffer.

Should we move fbuffer and buffer_size from foe_file_cfg to foe_cfg?

[qiayuanl]

A pull request is opened: #174