source: fix bootloader binary names.

When copy-pasting the instructions, we see:

$ fastboot flash bootloader u-boot-kvim3_ab.bin fastboot: error: cannot load 'u-boot-kvim3_ab.bin': No such file or directory

replace - by _ to fix the bootloader binary name.

Signed-off-by: Mattijs Korpershoek mkorpershoek@baylibre.com