В Buildroot:
make menuconfig

Toolchain --->
Enable toolchain locale/i18n

System configuration --->
(C en_US) Locales to keep
(ru_RU.UTF-8) Generate locale data
[*] Enable Native Language Support (NLS)

Filesystem and flash utilities --->
[*] ntfs-3g

Target packages --->
Hardware handling --->
[*] kbd

Target packages --->
System tools --->
[*] coreutils

В ядре:
File systems --->
-*- Native language support --->
<*> Codepage 866 (Cyrillic/Russian)
<*> Windows CP1251 (Bulgarian, Belarusian)
<*> NLS UTF-8
<*>ASCII (United States)
<*> NLS KOI8-R (Russian)

Все перепроверил еще на раз и пересобрал buildroot.

В profile прописал export LANG=ru_RU.UTF-8

# echo "Это образец текста" > файл.txt
# ls
rc.local ''$'\364\340\351\353''.txt получил '\364\340\351\353' вместо 'файл'

Settings ->
[*] Enable locale support (system needs locale for this to work)
[*] Support Unicode
[ ] Use libc routines for Unicode (else uses internal ones)
[*] Check $LC_ALL, $LC_CTYPE and $LANG environment variables
(63) Character code to substitute unprintable characters with
(0) Range of supported Unicode characters
[ ] Allow zero-width Unicode characters on output
[*] Allow wide Unicode characters on output
[ ] Bidirectional character-aware line input
[ ] Make it possible to enter sequences of chars which are not Unicode

root@192.168.1.2's password:
# ls
rc.local
# echo "Это образец текста" > файл.txt
# ls
rc.local ''$'\321\204\320\260\320\271\320\273''.txt'
# cat файл.txt
Это образец текста
#