lpac: add uqmi backend

lpac: add uqmi backend

This adds an additional OpenWrt specific backend to use lpac with the
uqmi tooling used by OpenWrt to manage QMI based modems.

This allows lpac to manage eUICC chips without the need for other,
potentially bigger, software using the installed modem.

Also set this backend as the new default, as users probably expect being
able to download profiles using their cellular modem.

Profile-Switching
-----------------
With some eUICC modem combinations you might require to powercycle the
UIM slot using uqmi after changing the active profile. To do this,
simply execute

$ uqmi -d /dev/cdc-wdm0 --uim-power-off --uim-slot=1
$ uqmi -d /dev/cdc-wdm0 --uim-power-on --uim-slot=1

Note
----
SM-DP+ might use encryption parameters which are incompatible with
mbedtls which cURL is by default compiled for.

This was observed when attempting to download a profile from
Vodafone DE.

If you encounter issues in that regard, try to install a version of
libcurl which is compiled with OpenSSL support.

Signed-off-by: David Bauer <redacted>