buildroot

Revert "nano: bump to version 2.7.5"

The bump causes build failure with musl, this new version uses with
undefined REG_STARTEND which is a glibc-ism that isn't available for
musl-based toolchains.
The introduction of this is to address
https://savannah.gnu.org/bugs/?50030 so it's not quite straightforward
to fix.

This reverts commit d0035543432edddee4dcb017ac4dd74832133e89.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>