sbc8548: enable and test CONFIG_SYS_GENERIC_BOARD

sbc8548: enable and test CONFIG_SYS_GENERIC_BOARD

Tested on the following baseline (note "dirty" since I enabled
ALT_BOOT in the config in order to use the alternate boot bank.)

Everything seems to work fine with no additional changes.  The
banner warning message is now gone.

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U-Boot 2014.10-rc1-00075-ge49f14af1349-dirty (Aug 14 2014 - 10:26:15)

CPU:   8548E, Version: 2.1, (0x80390021)
Core:  e500, Version: 2.2, (0x80210022)
Clock Configuration:
       CPU0:990  MHz,
       CCB:396  MHz,
       DDR:198  MHz (396 MT/s data rate), LBC:99   MHz
L1:    D-cache 32 KiB enabled
       I-cache 32 KiB enabled
I2C:   ready
DRAM:  Detected UDIMM
    SDRAM: 128 MiB
256 MiB (DDR2, 64-bit, CL=3, ECC off)
Flash: 72 MiB
L2:    512 KiB enabled
*** Warning - bad CRC, using default environment

PCI: Host, 64 bit, 66 MHz, sync, arbiter
  00:01.0     - 8086:1026 - Network controller
PCI1: Bus 00 - 00

PCIe1: Root Complex, x1 gen1, regs @ 0xe000a000
  02:00.0     - 1148:9e00 - Network controller
PCIe1: Bus 01 - 02
In:    serial
Out:   serial
Err:   serial
Net:   eTSEC0 [PRIME], eTSEC1
Hit any key to stop autoboot:  0
=> ver

U-Boot 2014.10-rc1-00075-ge49f14af1349-dirty (Aug 14 2014 - 10:26:15)
powerpc-linux-gcc (GCC) 4.5.2
GNU ld (GNU Binutils) 2.21
=>

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Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Reviewed-by: York Sun <yorksun@freescale.com>