Add mimxrt1060_evkb to all samples along with ethernet support

[szczys]

• Add Ethernet to common samples to work with this, and all ethernet-based boards.
• Add board-specific conf file
• Add include and check for golioth ethernet Kconfig symbol

This PR follows the example of samples/common/wifi, adding an Ethernet option to handle the needed DHCP operations for the connection type.

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[szczys]

After the changes this builds for the mimxrt1060_evkb, but not for other configurations that don't have CONFIG_NET_DHCPV4 selected. I'll be a little more clever and push some changes when I figure this out.

[mniestroj]

I guess you should include zephyr/net/dhcpv4.h to fix this build issue.

[mniestroj]

@szczys Nit: for consistency please use lowercase samples in commit messages, so that grepping git history with something like git log --grep=samples: would still be possible (with your commits included).

[szczys]

I added https://github.com/golioth/golioth-zephyr-sdk/pull/271/commits/44a567106135972c070af681f8da3bd8b6b084ce to this PR to change the way we call network helper functions from samples:

• net_config() is run by all samples but will be empty unless other symbols like CONFIG_GOLIOTH_SAMPLE_WIFI are selected
• This is not a breaking change, existing apps that call wifi_connect() are still able to do so
• Future additions to net_config() can be made without changing the samples (new symbols can be added in the \<board>.conf)

One question I have is whether I actually need to make the CONFIG_GOLIOTH_SAMPLE_DHCP_START dependent on NET_L2_ETHERNET. I need it for this board (and other Ethernet connections) but I don't know if there's a future use case for DHCP when ethernet will not be selected.