buildman: Improve [make-flags] section parser to allow quoted strings

The parser responsible for the '[make-flags]' section in
the '.buildman' settings file is currently not able to
handle quoted strings, as given in the sample bellow:

[make-flags]
qemu_arm=HOSTCC="cc -isystem /add/include" HOSTLDFLAGS="-L/add/lib"

This patch replaces the simple string splitter based on the <space>
delimiter with a regex tokenizer that preserves spaces inside double
quoted strings.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>