board/svm_sc8xx/svm_sc8xx.c: Fix GCC 4.6 build warning

Fix:
svm_sc8xx.c: In function 'checkboard':
svm_sc8xx.c:83:6: warning: variable 'board_type' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>