m68k: Fix relocation errors in start.S

When the environment sectors in the flash are big, one get those errors :

 mcf547x_8x/start.S:173: relocation truncated to fit: R_68K_PC16 against
  symbol `cpu_init_f' defined in .text section in libmcf547x_8x.a(cpu_init.o)
 mcf547x_8x/start.S:174: relocation truncated to fit: R_68K_PC16 against
  symbol `board_init_f' defined in .text section in libm68k.a(board.o)

Fix that.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Cc: "Jin Zhengxiong-R64188" <R64188@freescale.com>
Cc: Jason Jin <jason.jin@freescale.com>