cmake: add new ${RIMAGE_TOP} variable #14903

Job	Run time
build-windows (imx8 imx8x imx8m)	11m 49s
build-linux (mnfst, imx8 imx8x imx8m)	6m 36s
build-windows (mtl)	8m 49s
LP64-WIP	3m 40s
manifest-check	50s
build-windows (tgl tgl-h)	9m 20s
build-linux (mnfst, mtl)	6m 24s
build-windows (-d, mtl)	9m 14s
build-linux (mnfst, lnl)	5m 16s
build-linux (mnfst, tgl tgl-h)	6m 47s
build-linux (zmain, imx8 imx8x imx8m)	6m 32s
build-linux (zmain, mtl)	5m 45s
build-linux (zmain, lnl)	6m 34s
build-linux (zmain, tgl tgl-h)	6m 36s
build-linux (-d, mnfst, mtl)	5m 29s
compare-linux-win	1m 26s
	1h 41m 7s

Provide feedback