lib: libc: minimal: Get rid of the bit (256-byte) charmap table

The charmap table used by strncasecmp() not only used precious 256
bytes of ROM, it also had wrong mappings outside the ASCII range
(123..218).

Rewrite strncasecmp() to call tolower() instead; might be a tiny wee
little bit slower than the current version, but it's not used in any
performance-sensitive parts of the code to justify the waste.

This reduces the ROM footprint for the ws_echo_server sample by ~224
bytes.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>