build-uefi.sh: add UEFI_EXTRA_OPTIONS

Add UEFI_EXTRA_OPTIONS so that the variant config can supply extra build
options to the makefile.

note: to define a symbol in the .dsc and .fdf files and in the code, you
need to add "-D" before the symbol, eg:

  UEFI_EXTRA_OPTIONS="-D EDK2_USE_GICV3"

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>