Some notes on the current beta version and serial console operation:
Pi Zero W
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-- Stock image gives you a handful of gibberish when power is first applied, nothing further, no login prompt
-- Edit config.txt and add dtoverlay=pi3-disable-bt so it applies to the pi zero W and not just the Pi3 which is the default. This looks like it has no effect at power on but if you wait for the boot to complete you will get a login prompt. No kernel or piCore messages during boot, just a console login, which does actually let you login.
-- Edit cmdline.txt and switch the order of the console= statements so the ttyAMA0 entry is after tty1 or delete the tty1 entry leaving just console=ttyAMA0,115200 and then you get piCore boot messages and a login prompt which lets you login.
Pi3 B+
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-- Stock image gives you a handful of gibberish when power is first applied, nothing further, no login prompt
-- Edit cmdline3.txt and change ttyS0 to ttyAMA0: console=ttyAMA0,115200. This appears to have no effect except I get one kernel message: "Under-voltage detected! (0x00050005)". In this case, having a crappy power supply at least had the benefit of showing that changing to ttyAMA0 had an effect. Beyond this single message (which actually appears twice during boot) there is no other output, no additional kernel/piCore messages and no login prompt.
-- Edit cmdline3.txt again and switch the order of the console= statements and now I see the under-voltage messages as well as the piCore boot messages but no login prompt. When I press enter it does cause my serial terminal to advance a line and if I type characters I see them echoed to the screen. The pi is fully booted (I verified this by hooking up a monitor and keyboard and I can login that way) but I never get a serial login prompt.
-- Edit config.txt and comment out "enable_uart=1" and you still see piCore boot messages and no login prompt as the previous test but you don't see the ssh key generation ascii art which I did see before commenting out "enable_uart=1".
Reading about the serial port usage on the Pi3 is confusing (to me). Even if I don't really get what's going on fully yet, it does seem that the default piCore configuration isn't really ideal for the Zero W or the Pi3. Looking at the default configuration, the intent appears to be that console messages get logged to the monitor and that a login will be available at both the local monitor/keyboard and via the serial GPIO pins. If that's the desired operation, in the case of the Zero W, you need to add dtoverlay=pi3-disable-bt to make that happen. For the Pi3 you have to change to ttyAMA0 to get any serial response at all but you never get a login prompt so really a serial login doesn't really work for me at all yet.