Default u-boot "bootargs" variable for kernel is following:

bootargs=quiet console=ttyS0,115200 ubi.mtd=UBI root=ubi:RFS ubi.mtd=UBILD rootfstype=ubifs rw EMAC_MEM=0x100000 DRAM_LEN=0x10000000 mtdparts=edb64M-nand:2432k@0x180000(MBOOT),2432k(MBOOTBAK),2m(UBILD),4m(BFN),6m(KL),54m(MSLIB),18176k(APP),896k(RTPM),-(UBI) norandmaps BRICK_TERMINATOR_SPI_STATUS_OFFSET=0005D000 LX_MEM=0x7000000 LX_MEM2=0x50000000,0xF600000 PM51_ADDR=0x7100000 PM51_LEN=0x10000 BOOTLOGO_IN_MBOOT ENV_VAR_OFFSET=0xBA000 ENV_VAR_SIZE=0x10000 ENV=NAND SECURITY=OFF

 I tried:

• removing quiet
• adding single init=/bin/sh

Without success. If you have any ideas how to modify kernel's boot arguments to gain root shell on uart please leave comment.