samples: hello: increase NET_BUF_TX_COUNT in Kconfig

[mniestroj]

NET_BUF_TX_COUNT was increased in nrf52840dk_nrf52840 overlay configuration file. Do this in Kconfig by defining default value. This has following advantages:

• only applied with cert-based authentication (where original issue is reproducible)
• applied for all esp-at boards (there is only one supported by board overlay, but potentially all ESP-AT boards might benefit from this change due to low-bandwidth nature of this WiFi interface)
• all board overlay files across all samples are the same (at least for nrf52840dk_nrf52840 board)

[github-actions[bot]]

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