tftp-hpa: bug fix on separated CR and LF

In ascii mode, if the CR and LF was separated into different transfer
blocks, This LF will be just dropped instead of replacing the previous
CR. Add a checking on the first character to fix it.

Signed-off-by: Zhang Xiao <xiao.zhang@windriver.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>