Fix parsing string parameters in BOARD_MKBOOTIMG_ARGS

The existing logic in common.py breaks string arguments incorrectly:
e.g. --para1 val1 --para2 "val2 is a string" will be output as:

'--para', 'val1, '--para2', 'val2' 'is' 'a' 'string'

This will cause mkbootimg command fails due to the invalid arguments
generated from the wrong parsing.

The patch fixes this issue to get:

'--para', 'val1, '--para2', 'val2 is a string'

Change-Id: Ia34ec357550f11ae9d6adc719d86a0c6a9099fbc
Signed-off-by: Jianxun Zhang <jianxun.zhang@intel.com>