This worked for me in an equivalent circumstance, where the m68k file could not be found:
Edit the .bash_profile file in your home directory (cygwin\home\[your name]\.bash_profile), to include the path line at top,
Use Notepad++ or a similar code editor to edit the file (seemingly NOT Windows WordPad or NotePad, which add in line endings). Notepad++ is freely available (for free) on the web.
Presumably, here for you, you would add the path line to the .bash_profile file,
As it was explained in a thread here, the path line change can be added into a different file, as noted in the Rockbox Cygwin install instructions, but that file would get overwritten with a new Cygwin version install; the solution here, recommended in the forums, is said to avoid that issue.
I hope this helps.