qt4: use extra variable for more QT_CONFIG_FLAGS fragments

qt4: use extra variable for more QT_CONFIG_FLAGS fragments

* qt4-embedded was forcing -DQT_KEYPAD_NAVIGATION which depends on feature-completer
* separate variable makes it easier to not enable QT_KEYPAD_NAVIGATION in some upper layer where we have disabled feature-completer

(From OE-Core rev: 0479242a18661cb7fc3d76d208c82fe6ae4378ce)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>