download this: https://www.dropbox.com/s/ytimsezu9cl61xx/synoboot_1.02b_ESXi_only.zip?dl=0
the only option available is ESXi, so you should not have problems regarding boot choice, it's for DS3615xs (i find has a better custom package support than DS3517xs).
- extract and upload synoboot.vmdk and synoboot.img in your datastore
- create a new VM (HW Version 11 or 13) dependent if you are on ESXi 6.0u2 or ESXi 6.5b
- operating system should be Other -> FreeBSD 64bit
- set bios to EFI
- set nic to VMXNET3, set the mac manual to the same as the one in the grub.cfg
- set scsi controller 0 to "LSI logic SAS"
- add sata controller 0 if not there yet present
- add existing hdd -> browse datastore -> add synoboot.vmdk
- edit this hdd to "independent - persistent" and sata 0:0 on controller 0
- add how many new scsi disk you like, thin provisioning - dependent
- save and poweron the VM
- from your browser, go to find.synology.com, it should pop-up, connect and install it (manual or from internet is the same), reboot after finish the setup
- create a new volume to your needs, if you want shr you have to modify synoinfo.conf
- download this zip for open-vm-tools https://www.dropbox.com/s/k4hc343gwdepiuf/open-vm-tools_bromolow-6.0_10.0.7-1.spk.zip?dl=0
- extract the zip, install the spk in package manager
- now ESXi should see that vm tools are installed and running
Pretty much done.