MA-12973 [AUTO] Skip serial init in board_init_f

MA-12973 [AUTO] Skip serial init in board_init_f

Serial init in board_init_f will cost much time, skip it here
because the serial will be initialized again in board_init_r
and it's more faster (after cache is enabled). We will miss
some logs before the serial is ready but it's ok for Android Auto.
This commit will save about 190ms on imx8qm.

Test: boot ok for both imx8qm and imx8qxp.

Change-Id: If6efdc19794aecda862f22b6fec7f7aba2005766
Signed-off-by: Ji Luo <ji.luo@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>