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.build b/meson.build
index a427bdb..aa6da2a 100644
--- a/meson.build
+++ b/meson.build
@@ -347,6 +347,7 @@
 
 if get_option('tests').allowed()
     subdir('test')
+    subdir('transport/serialbridge')
 endif
 
 install_subdir(
@@ -355,3 +356,6 @@
     strip_directory: false,
     exclude_files: '*.cpp',
 )
+
+# HW Transport
+subdir('transport')