You can use the cfsetispeed() and cfsetospeed() functions from termios in a user-space application:
http://en.wikibooks.org/wiki/Serial_Programming/termios
You can also use the "stty" command. Here is a useful reading about the UART serial port: http://www.tldp.org/HOWTO/Serial-HOWTO.html
Regarding U-Boot, the serial driver is inside <u-boot_dir>/drivers/serial.serial.c.
Best regards,
Miroslav