u-boot: Building in LC_ALL=en_US.UTF8 env will lost commands

If the u-boot is built in the LC_ALL=en_US.UTF8 Linux environment,
any command typing on the u-boot shell will be regarded as unknown
command.  Take Accton AS4600_54T as an example:

    U-Boot 2013.01.01-g8090123 (Nov 11 2014 - 18:16:06) - ONIE 2014.11.11

    CPU0:  P2020E, Version: 2.1, (0x80ea0021)
    Core:  E500, Version: 5.1, (0x80211051)
    Clock Configuration:
           CPU0:1000 MHz, CPU1:1000 MHz,
           CCB:500  MHz,
           DDR:300  MHz (600 MT/s data rate) (Asynchronous), LBC:31.250 MHz
    L1:    D-cache 32 kB enabled
           I-cache 32 kB enabled
    Board: Accton AS4600_54T (cpld_ver: 0.4)
    I2C:   ready
    DRAM:  fixed_sdram Configuring DDR for 600 MT/s data rate
    2 GiB (DDR3, 64-bit, CL=6, ECC on)
    Flash: 8 MiB
    L2:    512 KB enabled
    EEPROM: TlvInfo v1 len=62
    PCIe1: Root Complex, x1, regs @ 0xff70a000
      01:00.0     - 14e4:b540 - Network controller
    PCIe1: Bus 00 - 01
    In:    serial
    Out:   serial
    Err:   serial
    Net:   eTSEC2 is in sgmii mode.
    eth0 [PRIME]
    EEPROM: TlvInfo v1 len=62
    Hit any key to stop autoboot:  0
    LOADER=> help
    Unknown command 'help' - try 'help'
    LOADER=> ver
    Unknown command 'ver' - try 'help'
    LOADER=> usb start
    Unknown command 'usb' - try 'help'
    LOADER=>

Reference:
- http://patchwork.ozlabs.org/patch/203247/