Fix build for mingw32 on windows ($$ expansion)

Fix build for mingw32 on windows ($$ expansion)

Make using mingw32 on windows fails when running grep "=y$$".
The command is expanded to grep "=y$ and the missing "
results in an error.

I don't expect a file config-devices.mak with =y somewhere in
the middle of a line (they are always at the end of the line),
so simplifying the regular expression to =y seems to be permitted.

This avoids problems with wrong expansion.

Signed-off-by: Stefan Weil <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>