[U-Boot] i.MX28 u-boot SPL fail

[Marco Cavallini]

Hi,
On a custom i.MX28 board I am using u-boot-2015.01.
The debug port DUART is not the same as i.MX28, so I changed these
settings in iomux.c:

const iomux_cfg_t iomux_setup[] = {
  /* DUART */
// MX28_PAD_PWM0__DUART_RX,
// MX28_PAD_PWM1__DUART_TX,

  /* DUART */
  /* Unconfigure BOOT ROM default DUART */
  MX28_PAD_PWM0__GPIO_3_16,
  MX28_PAD_PWM1__GPIO_3_17,
  /* Configure DUART on alternate pins */
  MX28_PAD_I2C0_SCL__DUART_RX,
  MX28_PAD_I2C0_SDA__DUART_TX,

I enabled #define CONFIG_SPL_SERIAL_SUPPORT and built uboot with :
make u-boot.sb
At last I can program the board only using MfgTools or 'mxsldr', or it
seems working.
The problem is that I get no messages on output debug port or some
random chars every 5, 10 boots

At this point I have some doubts and questions:
- is the u-boot.sb suitable to be run from MfgTools?
- should I expect any char on the serial debug port?
- do I have to use the 'elftosb' tool to convert my binary? (README says not)

TIA
--
Marco