pc_x86_64_bios_defconfig: increase ext2 filesystem size

Since commit c6bca8ce removed autocalculation of the ext2 filesystem
size, the default size is now set to 60MB. However, this is too small
for pc_x86_64_bios_defconfig. Indeed, the ext2 filesystem contains the
kernel (4MB), the wireless modules (4MB), all firmware for wireless
modules (40MB), and the wifi userspace (9MB) and the udev hwdb (5MB)
which brings the total to 70MB.

Increase the filesystem size to 120000K, which is a nice and round
number and leaves enough space for overhead on a 128MB flash drive.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>