Bluetooth: Controller: Use a different flag for speed optimization

The bluetooth controller has been using the flag '-Ofast' to keep
within a real-time limit. There are two problems with this; firstly,
when a project should be optimized for size it is standard to use -O2,
not -Ofast.

Secondly, optimization flags have been deemed to be non-portable, so
instead of directly using "-Ofast" we should use the intent-macro
OPTIMIZE_FOR_SPEED_FLAG to ensure toolchain portability.

Testing has shown that we are still within the real-time limit when
changing from -Ofast to -O2. -Ofast is about 1us and 1% faster, but
increases the code size by 13kB (5% of the available flash on a
nRF51).

Since the slowdown is comparatively small compared to the code size
increase we have decided to use -O2 in place of -Ofast.

Other optimization combinations were also measured and their results
can be seen below:

-Ofast in BLE Controller and in #pragma in entropy driver.

        Memory region         Used Size  Region Size  %age Used
                   FLASH:      138920 B       256 KB     52.99%

        [bt] [INF] encode_control: l: 6, 6, 7; t: 75, 45, 132.

-O2 in BLE Controller and in #pragma in entropy driver.

        Memory region         Used Size  Region Size  %age Used
                   FLASH:      125840 B       256 KB     48.00%

        [bt] [INF] encode_control: l: 6, 6, 7; t: 75, 51, 133

-O3 in BLE Controller and in #pragma in entropy driver.

        Memory region         Used Size  Region Size  %age Used
                   FLASH:      138920 B       256 KB     52.99%

        [bt] [INF] encode_control: l: 6, 6, 7; t: 75, 50, 132.

No extra CFLAGS for BLE and entropy driver (pragmas removed), using
CONFIG_SIZE_OPTIMIZATIONS=y

        Memory region         Used Size  Region Size  %age Used
                   FLASH:      120124 B       256 KB     45.82%

        [bt] [ERR] isr_rx_conn: assert: '!radio_is_ready()' failed

No extra CFLAGS for BLE and entropy driver (pragmas removed), using
CONFIG_SPEED_OPTIMIZATIONS=y

        Memory region         Used Size  Region Size  %age Used
                   FLASH:      138004 B       256 KB     52.64%

        [bt] [INF] encode_control: l: 6, 6, 7; t: 61, 51, 130.

NB: RAM usage differences were insignificant.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>