Check INSTALLED_SYSTEMIMAGE_TARGET to build INSTALLED_QEMU_SYSTEMIMAGE

This change checks if INSTALLED_SYSTEMIMAGE_TARGET
is defined to generate INSTALLED_QEMU_SYSTEMIMAGE
to improve debugging.

Bug: 79601115
Test: make -j
Change-Id: Ia1aad7dbaab1d2de11a1bc3ffe7c7ae714f5f6eb
Signed-off-by: Roman Kiryanov <rkir@google.com>