nimble/phy: Implement software T_IFS control
nimble/phy: Implement software T_IFS control

We use hw controlled T_IFS which does not work well on LE Coded PHY and
we're way off required 150 usecs so it is not possible to have reliable
connection with other devices (...or impossible to connect at all).

To fix this we need to control T_IFS in sw which means we do not use
DISABLED_TXEN and DISABLED_RXEN shorts and schedule TXEN and RXEN using
TIMER0. The radio timings in nRF specification are either missing or
inaccurate so they were measured instead for each PHY.

With new implementation measured T_IFS on each PHY is 150 +/- 1 usec
which is on par with TIMER0 resolution.

mlaz/mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,IMGJulian/incubator-mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/apache-mynewt-core