tools: Build mkeficapsule tool by default if EFI_LOADER is set

tools: Build mkeficapsule tool by default if EFI_LOADER is set

Trigger the building of the mkeficapsule tool if EFI_LOADER is enabled.
Previously it was triggered on EFI_CAPSULE_ON_DISK, but mkeficapsule is
needed when a capsule is being generated for a bootloader stage, not just
from the stage applying them. EFI_LOADER is a more accurate approximation
of when a capsule may need to be generated.

Signed-off-by: Jonathan Humphreys <j-humphreys@ti.com>