romlib: Improve compilation flags definition

* Optimization flags were only provided for debug build.
* Set optimisation level to -O1
* Remove CFLAGS which is never used for romlib
* Remove the ignored -g flag from LDFLAGS

Change-Id: Id4b69026d8a322ed4cb0acf06c350f13d31571ad
Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>