test_fs: Add exfat tests

test_fs: Add exfat tests

Add tests for the exfat filesystem. These tests are largely an
extension of the FS_GENERIC tests with the following notable
exceptions.

The filesystem image for exfat tests is generated using combination
of exfatprogs mkfs.exfat and python fattools. The fattols are capable
of generating exfat filesystem images too, but this is not used, the
fattools are only used as a replacement for dosfstools 'mcopy' and
'mdir', which are used to insert files and directories into existing
fatfs images and list existing fatfs images respectively, without the
need for superuser access to mount such images.

The exfat filesystem has no filesystem specific command, there is only
the generic filesystem command interface, therefore check_ubconfig()
has to special case exfat and skip check for CONFIG_CMD_EXFAT and
instead check for CONFIG_FS_EXFAT.

Signed-off-by: Marek Vasut <marex@denx.de>