uboot-envtools: Fix format of autogenerated sectors

uboot-envtools: Fix format of autogenerated sectors

The sector number must be stored in hex. Otherwise, the number (like 16)
will be parsed as hex and any write to the partition will end up with an
error like:

  MTD erase error on /dev/mtd5: Invalid argument

Fixes: 9adfeccd8415 ("uboot-envtools: Add support for IPQ806x AP148 and DB149")
Fixes: 54b275c8ed3a ("ipq40xx: add target")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@fungible.com>
(cherry picked from commit 8d3e932b65b59ab7231cb5440866eb975bd150ea)