Modify variable ROOTFS_DIR to call both build-native and build-managed (dotnet/corefx#10407)

Modify variable ROOTFS_DIR to call both build-native and build-managed and stop passing Platform=arm-soft for build-managed


Commit migrated from https://github.com/dotnet/corefx/commit/bd37ac4d674d493f33f7b66cf37207e608404d27

src/libraries/scripts/arm32_ci_script.sh

@@ -146,9 +146,8 @@ function cross_build_corefx {
     git am < "$__ARMRootfsMountPath"/dotnet/setenv/corefx_cross.patch
 
     #Cross building for emulator rootfs
-    ROOTFS_DIR="$__ARMRootfsMountPath" CPLUS_INCLUDE_PATH=$LINUX_ARM_INCPATH CXXFLAGS=$LINUX_ARM_CXXFLAGS
-    BUILD_NATIVE="$ROOTFS_DIR" ./build-native.sh -buildArch=$__buildArch -$__buildConfig -- cross $__verboseFlag
-    BUILD_MANAGED="$ROOTFS_DIR" ./build-managed.sh -buildArch=$__buildArch -$__buildConfig -skipTests
+    ROOTFS_DIR="$__ARMRootfsMountPath" CPLUS_INCLUDE_PATH=$LINUX_ARM_INCPATH CXXFLAGS=$LINUX_ARM_CXXFLAGS ./build-native.sh -buildArch=$__buildArch -$__buildConfig -- cross $__verboseFlag
+    ROOTFS_DIR="$__ARMRootfsMountPath" CPLUS_INCLUDE_PATH=$LINUX_ARM_INCPATH CXXFLAGS=$LINUX_ARM_CXXFLAGS ./build-managed.sh -$__buildConfig -skipTests
 
     #Reset the code to the upstream version
     (set +x; echo 'Rewinding HEAD to master code')