Revert "wpa_supplicant: VERSION_STR_POSTFIX is now defined to 'git describe'"

This reverts commit 056fb2b3c188f90337534623ff9b36c53de7ae9c.

This is to fix the Android build warning  message "fatal: nothing to describe",
Since we don't use git tags in omapzoom.

Reverting back to PLATFORM_VERSION.

Change-Id: Ie6b4e6ba2f6a7327cbe01db268a9c18199aecc79
Signed-off-by: Vishal Mahaveer <vishalm@ti.com>