boot/grub2: backport parallel-build fix

There seems to still be a similar parallel build issue, but with this
patch applied, it occurs much less frequently: from a 1/3rf failure
rate, I am now experiencing failures under the 1/10th mark.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>