procd: nand: dont rely on boardname in nand_upgrade_tar

Kernel and rootfs in a subdirectory matching the userspace boardname,
was intended to use a single sysupgrade-tar archive for multiple boards
with different kernel/rootfs images. This feature was never used.

Use the first found directory in the tar archive instead of relying on
a directory named according to the userspace boardname.

It allows to change the boardname without adding another compatibility
layer - using the nand_board_name() function - for (sub)targets using
the metadata based image validation in favour to
nand_do_platform_check().

Signed-off-by: Mathias Kresin <dev@kresin.me>