build: allow specifying a custom linker script relative to project

Build system will first check if the linker script is found relative to
the project directory, and if not, it will consider it to be an absolute
path, as before.

Change-Id: Idfc7e55febbc8c197643ca514dc01c3e2262712a
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>