fs/erofs: enable support for reproducible build

By default, mkfs.erofs will sue the current date to set some metadata
in the filesystem it generates, and will also use generate a random UUID
for that filesystem. This is not reproducible.

When BR2_REPRODUCIBLE is selected, set the filesystem timestamps to
$SOURCE_DATE_EPOCH, and the filesystem UUID to the nil uuid (as good
as any other arbitrary one).

Signed-off-by: Casey Reeves <casey@xogium.me>
[yann.morin.1998@free.fr:
  - one multi-line assignment, not two
  - slightly rephrase the commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>