Support a raw bootconfig file as input to vendor-bootconfig.img

Make variables can only get us so far...
This allows for more of the supported input formats for boot config,
like spaces in the values.

Ex:
foo=this will work in a file

If that is in a file, it will be added to the vendor-bootconfig.img
as-is. If it is in the BOARD_BOOTCONFIG variable, it gets chopped up to
look like multiple different arguments like:

foo=this
will
work
in
a
file

Test: build with/without BOARD_BOOTCONFIG and BOARD_BOOTCONFIG_FILE
Test: verify contents of vendor-bootconfig.img in all 4 combos
Test: verify "bootconfig" kernel cmdline arg doesn't show up in
vendor_boot.img when neither variable is set
Bug: 374651835

Change-Id: If59b1a9c968799bacbd4d88e0191e6bddc81a361