The 50 ms TX mutex timeouts introduced in commit b0a0feb cause
dropped messages when used with the shell log backend. This
commit makes time timeout configurable using the
CONFIG_SHELL_TX_TIMEOUT_MS Kconfig option and makes the
default timeout longer when shell log backend is enabled.
Link: #90215
Signed-off-by: Tommi Kangas <tommi.kangas@nordicsemi.no>