[nrfconnect] Enable FPU and adjust stack sizes (#15797)

FPU support is necessary to be able to use pre-compiled
OpenThread libraries. However, when FPU and stack guard
are enabled, BLE RX thread stack must be increased, too.
By the way, remove the OpenThread stack size setting as
it's set correctly, by default.

config/nrfconnect/app/sample-defaults.conf

@@ -25,6 +25,7 @@ CONFIG_MATTER_LOG_LEVEL_DBG=y
 CONFIG_PRINTK_SYNC=y
 CONFIG_ASSERT=y
 CONFIG_HW_STACK_PROTECTION=y
+CONFIG_FPU=y
 CONFIG_SHELL=y
 
 # Enable getting reboot reasons information
@@ -63,6 +64,7 @@ CONFIG_BT_MAX_CONN=1
 CONFIG_BT_L2CAP_TX_MTU=247
 CONFIG_BT_BUF_ACL_RX_SIZE=251
 CONFIG_BT_BUF_ACL_TX_SIZE=251
+CONFIG_BT_RX_STACK_SIZE=1120
 
 # Enable NFC support
 CONFIG_CHIP_NFC_COMMISSIONING=y
@@ -76,7 +78,6 @@ CONFIG_OPENTHREAD_FTD=n
 CONFIG_OPENTHREAD_SLAAC=y
 CONFIG_OPENTHREAD_MTD_NETDIAG=y
 CONFIG_OPENTHREAD_MANUAL_START=y
-CONFIG_OPENTHREAD_THREAD_STACK_SIZE=6144
 CONFIG_OPENTHREAD_NUM_MESSAGE_BUFFERS=64
 
 # Enable Thread 1.2 features