uboot-tools: envtools: ipq806x: ignore case of partition name

uboot-tools: envtools: ipq806x: ignore case of partition name

The qcomsmem MTD partition parser converts all partition names to
lower case while the vendor solution uses upper case names, which
often made their way into OpenWrt as labels in 'fixed-partitions'
(probably due to contributors and reviewers being unaware of the
qcomsmem parsers).

Use case-insensitive matching of the 'APPSBLENV' name to make
ubootenv_mtdinfo() work in both cases.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>