Support IPMI Serial Transport

Support IPMI Serial interface (basic mode) based on Sec 14,
IPMI 2.0 spec. Serial transport tested with meta-evb-arm
platform with socat and ipmitool.

HW transport can be selected by meson configure flasg.

Tested:

1. socat -x pty,raw,echo=0,link=/tmp/ttyHOST2BMC tcp:localhost:5066
2. ipmitool -I serial-basic -D /tmp/ttyHOST2BMC:115200 mc info

    Device ID                 : 0
    Device Revision           : 0
    Firmware Revision         : 2.17
    IPMI Version              : 2.0
    Manufacturer ID           : 0
    Manufacturer Name         : Unknown
    Product ID                : 0 (0x0000)
    Product Name              : Unknown (0x0)
    Device Available          : no
    Provides Device SDRs      : no
    Additional Device Support :
    Aux Firmware Rev Info     :
        0x00
        0x00
        0x00
        0x00

Change-Id: I2a56390022fce7a974ed75ed6a72ad3fffed9bb6
Signed-off-by: John Chung <john.chung@arm.com>
diff --git a/meson.options b/meson.options
index 6d75e73..5a21171 100644
--- a/meson.options
+++ b/meson.options
@@ -243,3 +243,11 @@
     type: 'feature',
     description: 'Support open-power specific functions',
 )
+
+# HW transport
+option(
+    'transport-implementation',
+    type: 'combo',
+    choices: ['null', 'serial'],
+    description: 'transport'
+)