Makefile: Fix @EARLY_PREFIX@ error

After executing 'make' command, systemd's service files are generated
from their *.in files. However, "@EARLY_PREFIX@" string in *.in files is
not converted properly. As a result, service files generated have a raw
"@EARLY_PREFIX@".

This patch fixes this error.

Signed-off-by: Sangjung Woo <sangjung.woo@samsung.com>