board/technologic/ts7680: fix bogus size in genimage.cfg

board/technologic/ts7680: fix bogus size in genimage.cfg

512B is not a correct size to express "512 bytes", and causes a
genimage failure:

  ERROR: Invalid size suffix 'B' in '512B'

To express "512 bytes", using just "512" is sufficient. With this
commit, genimage works fine, and we indeed have a 512 bytes unused
partition:

$ fdisk -l output/images/sdcard.img
Disk output/images/sdcard.img: 60 MiB, 62915584 bytes, 122882 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x00000000

Device                    Boot Start    End Sectors  Size Id Type
output/images/sdcard.img1          1      1       1  512B  0 Empty
output/images/sdcard.img2          2 524289  524288  256M 83 Linux

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/88314963

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>