Add a standalone grep application under apps/system/grep/. It uses the
NuttX libc regex library (regcomp/regexec) to support -i, -n, -v, -r,
-H and -h options plus stdin input and recursive directory search.
SYSTEM_GREP depends on LIBC_REGEX, so the regex library must be enabled
first -- CONFIG_SYSTEM_GREP only becomes selectable once LIBC_REGEX is
turned on.
Signed-off-by: Junbo Zheng <zhengjunbo1@xiaomi.com>
