pxe: simplify label_boot()

Coverity CID 131256 indicates a possible buffer overflow in label_boot().
This would only occur if the size of the downloaded file would exceed 4
GiB. But anyway we can simplify the code by using snprintf() and checking
the return value.

Addresses-Coverity-ID: 131256 ("Security best practices violations (STRING_OVERFLOW)")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Artem Lapkin <email2tema@gmail.com>