yaffs u-boot: Add new ycheck command, fix return types and modify filesize

yaffs u-boot:  Add new ycheck command, fix return types and modify filesize

ycheck filename [DIR|REG] checks if the file exists and is of the correct type
All functions now return a value that can be tested by $?
filesize env vaiable is now set by a read

Signed-off-by: Charles Manning <cdhmanning@gmail.com>