build/core: Use better default compiler flags

Use -O3 for the compiler, -O2 for the linker

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>

Conflicts:
	core/combo/select.mk
build: allow use of openjdk and other versions of make

Change-Id: I2e3c7f89ce51888afb9cda30eeaf6d757b057e2e
Squas cm build commits