rtt: Improve buffer allocation and handling #1475

Job	Run time
call-ci / build (armv7a7-imx6ull-evk)	1m 16s
call-ci / build (armv7a9-zynq7000-qemu)	1m 26s
call-ci / build (armv7a9-zynq7000-zedboard)	1m 12s
call-ci / build (armv7a9-zynq7000-zturn)	58s
call-ci / build (armv7m4-stm32l4x6-nucleo)	1m 4s
call-ci / build (armv7m7-imxrt105x-evk)	1m 9s
call-ci / build (armv7m7-imxrt106x-evk)	1m 12s
call-ci / build (armv7m7-imxrt117x-evk)	1m 22s
call-ci / build (host-generic-pc)	1m 1s
call-ci / build (ia32-generic-pc)	1m 8s
call-ci / build (ia32-generic-qemu)	1m 2s
call-ci / build (riscv64-generic-qemu)	56s
call-ci / build (riscv64-generic-spike)	59s
call-ci / build (sparcv8leon3-gr716-mini)	1m 5s
call-ci / test EMU (armv7a9-zynq7000-qemu)	8m 17s
call-ci / test HW (armv7a7-imx6ull-evk)	6m 32s
call-ci / test HW (armv7m7-imxrt106x-evk)	9m 54s
call-ci / test EMU (host-generic-pc)	51s
call-ci / test EMU (ia32-generic-qemu)	6m 22s
call-ci / test HW (armv7m7-imxrt117x-evk)	2m 45s
call-ci / test EMU (riscv64-generic-qemu)	5m 24s
call-ci / test HW (armv7a9-zynq7000-zedboard)	6m 56s
call-ci / test HW (armv7m4-stm32l4x6-nucleo)	11m 35s
call-ci / tests-summary	17s
call-ci / send-notification	0s
	1h 14m 43s

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