Add uartN_tx_margin() and uartN_tx_active() with uartN_tx_size() removal.

git-svn-id: http://ninja-scan-light.googlecode.com/svn/trunk@159 94d2995a-3457-b8a5-7510-40b377e6a921
Change uart1 buffer overflow check procedure.

git-svn-id: http://ninja-scan-light.googlecode.com/svn/trunk@160 94d2995a-3457-b8a5-7510-40b377e6a921
Correct comment.

git-svn-id: http://ninja-scan-light.googlecode.com/svn/trunk@161 94d2995a-3457-b8a5-7510-40b377e6a921