zephyr/cmake: add new ${RIMAGE_TOP} constant #756

Job	Run time
build-linux (mnfst, imx8 imx8x imx8m)	5m 32s
build-windows (imx8 imx8x imx8m)	13m 14s
LP64-WIP	3m 42s
build-linux (mnfst, mtl)	5m 40s
build-windows (mtl)	7m 40s
build-linux (mnfst, lnl)	5m 23s
build-windows (tgl tgl-h)	7m 42s
manifest-check	48s
build-linux (mnfst, tgl tgl-h)	6m 49s
build-windows (-d, mtl)	7m 52s
build-linux (zmain, imx8 imx8x imx8m)	5m 32s
build-linux (zmain, mtl)	5m 20s
build-linux (zmain, lnl)	5m 17s
build-linux (zmain, tgl tgl-h)	6m 36s
build-linux (-d, mnfst, mtl)	5m 30s
compare-linux-win	1m 14s
	1h 33m 51s

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