Kconfig: Use 'default' instead of 'def_bool' in Kconfig.defconfig files

This can help find unused symbols. Those end up without a type if
'default' is used instead of 'def_bool', which generates a warning.

Search for "Kconfig.defconfig" in
https://docs.zephyrproject.org/latest/application/kconfig-tips.html

 for
a longer explanation.

Keep the 'def_bool' for the following symbols, which seem to be
deliberately defined only in Kconfig.defconfig files:

 - ALTERA_AVALON_I2C
 - ALTERA_AVALON_MSGDMA
 - ALTERA_AVALON_PIO
 - ALTERA_AVALON_QSPI
 - ALTERA_AVALON_SYSID
 - CLOCK_CONTROL_IMX_CCM
 - CPU_EM4_DMIPS
 - CPU_EM4_FPUDA
 - CPU_EM4_FPUS
 - FP_FPU_DA
 - I2C_GECKO

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>