Previously VFS driver for UART could only use simple non-blocking
functions to read from and write to the UART. UART driver provides more
complex blocking and interrupt-driven functions, which can be used
instead.
This commit adds optional support for using UART driver's functions.
Also added is a more flexible mechanism for configuring newline
conversion rules on input and output.
This commit also fixes a bug that all UARTs shared one static variable
used as a character buffer in newline conversion code. This variable is
changed to be per-UART.
