Refactored code to support INSTANCES as string

Modified code in meson build to support INSTANCES flag as string
type which is passed from machine layer bb file.

Tested: Tested and verified the INSTANCES value.

Signed-off-by: Jayashree Dhanapal <jayashree-d@hcl.com>
Change-Id: I91a7610157c30abf2cbc4783350d8b0a744a6ba9
5 files changed