x86: Fix `make term` for BOARD=qemu-i386

Fixes #2184.

QEMU has changed its command line arguments.
Merge pull request #2185 from Kijewski/issue-2184

x86: QEMU has changed its command line arguments
make: change escaped quotation marks

The '""' pattern is used so that the variable can be processed as a string
after the preprocessors evaluation. Change to \"\" instead, so that single
quotes can be used externally. The extra quotation marks are unneeded.

Related to #2213
Merge pull request #2227 from LudwigOrtmann/make-escape

make: change escape sequence