AP_BoardConfig: support flow control control on UARTs 6->8

[andyp1per]

Currently four boards support SERIAL7 with flow control, but there is no support for controlling this via BRD_SER7_RTSCTS. This fixes the problem

[andyp1per]

Durandal

 Binary Name      Text [B]     Data [B]     BSS (B)      Total Flash Change [B] (%)      Flash Free After PR (B)
---------------  -----------  -----------  -----------  ----------------------------  -------------------------
blimp            0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      591888
ardurover        0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      291156
ardusub          0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      347128
arducopter-heli  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      135980
arduplane        0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      149660
arducopter       0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      134380
antennatracker   0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      614092

[IamPete1]

Could we add a #error somewhere to remind us to add these if we add support for more serial ports? We really should move them over to serial options....