kconfig: nconf: Format and print 'line' without a temporary copy

Use "%.*s" as the format specifier and supply the 'line' length 'len' to
mvwprintw() to format and print each line without making a temporary
copy. Remove the temporary buffer.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250811161650.37428-2-thorsten.blum@linux.dev


Signed-off-by: Nathan Chancellor <nathan@kernel.org>