net: lwip: extend wget to support CA (root) certificates

net: lwip: extend wget to support CA (root) certificates

Add the "cacert" (Certification Authority certificates) subcommand to
wget to pass root certificates to the code handling the HTTPS protocol.
The subcommand is enabled by the WGET_CACERT Kconfig symbol.

Usage example:

 => dhcp
 # Download some root certificates (note: not authenticated!)
 => wget https://cacerts.digicert.com/DigiCertTLSECCP384RootG5.crt
 # Provide root certificates
 => wget cacert $fileaddr $filesize
 # Enforce verification (it is optional by default)
 => wget cacert required
 # Forget the root certificates
 => wget cacert 0 0
 # Disable verification
 => wget cacert none

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>