px5g-mbedtls: Fix permission of private key

px5g-mbedtls: Fix permission of private key

Store the private key with read and write permission for the user only
and not with read permissions for everyone. This converts the
write_file() function from fopen() to open() because open allows to
specify the permission mask of the newly created file. It also adds and
fixes some existing error handling.

OpenSSL does this in the same way already.

With this change it looks like this:
root@OpenWrt:/# ls -al /etc/uhttpd.crt /etc/uhttpd.key
-rw-r--r--    1 root     root           519 Nov  6 22:58 /etc/uhttpd.crt
-rw-------    1 root     root           121 Nov  6 22:58 /etc/uhttpd.key

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>