Use the label provided when formating a dos partition

Previously the bootimg-pcbios wic plugin was not respecting
the --label option provided from the wks file. The plugin
was setting the label to 'boot'. With this fix, the --label
option is use. If no option are specified, then the default
is 'boot'.

Signed-off-by: jbouchard <jeanbouch418@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>