Add U-Boot adjustments to the libexfat code and integrate
the result into U-Boot filesystem layer. This provides full
read-write exfat support for U-Boot available via generic
filesystem interface.

FS_DIRENT_NAME_LEN is increased to 1024 in case exfat is
enabled, because EXFAT can use UTF16 names, which do not
fit into current FS_DIRENT_NAME_LEN. To avoid affecting
every configuration, increase FS_DIRENT_NAME_LEN only in
case EXFAT is enabled.

Example usage via sandbox, assuming disk.img with one exfat partition:

Drive info:
$ ./u-boot -Tc 'host bind 0 ../disk.img ; host info 0'
dev       blocks  blksz label           path
  0       262144    512 0               ../disk.img

List files:
$ ./u-boot -Tc 'host bind 0 ../disk.img ; ls host 0:1 /api'
      475   Kconfig
      230   Makefile
     1873   README
     ...
10 file(s), 0 dir(s)

Load and checksum a file:
$ ./u-boot -Tc 'host bind 0 ../disk.img ; load host 0:1 $loadaddr .config ; \
                crc32 $loadaddr $filesize'
56724 bytes read in 1 ms (54.1 MiB/s)
crc32 for 00000000 ... 0000dd93 ==> b2e847c9

$ crc32 .config
b2e847c9

Load .config file to RAM, store the file into FS as /newconfig,
load the /newconfig into RAM and checksum the file:
$ ./u-boot -Tc 'host bind 0 ../disk.img ; load host 0:1 $loadaddr .config ; \
		save host 0:1 $loadaddr /newconfig $filesize ; \
		load host 0:1 0x10000 /newconfig ; \
		crc32 0x10000 $filesize'
56724 bytes read in 1 ms (54.1 MiB/s)
56724 bytes written in 0 ms
56724 bytes read in 0 ms
crc32 for 00010000 ... 0001dd93 ==> b2e847c9

Remove file 3.txt and create new directory /newdir:
$ ./u-boot -Tc 'host bind 0 ../disk.img ; ls host 0:1 / ; \
                rm host 0:1 3.txt ; mkdir host 0:1 /newdir ; \
		ls host 0:1 /'
...
        0   1.txt
        0   2.txt
        0   3.txt
        0   4.txt
        0   5.txt

7 file(s), 4 dir(s)
...
        0   1.txt
        0   2.txt
            newdir/
        0   4.txt
        0   5.txt

6 file(s), 5 dir(s)

Acked-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Marek Vasut <marex@denx.de>