For Blackfin, C symbols are prefixed with '_'.

If CONFIG_CMD_MAGICVAR is enabled for Blackfin, the following error occurs:

commands/built-in.o: In function `do_magicvar':
commands/magicvar.c:(.text+0x3326): undefined reference to `__barebox_magicvar_start'
commands/magicvar.c:(.text+0x332a): undefined reference to `__barebox_magicvar_end'
commands/magicvar.c:(.text+0x332e): undefined reference to `__barebox_magicvar_start'
commands/magicvar.c:(.text+0x3332): undefined reference to `__barebox_magicvar_end'
make: *** [barebox] Error 1

Add '_' to __barebox_magicvar_start and __barebox_magicvar_end,
like the other symbols in this linker script.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>