rpm: really take DEPENDENCIES into account

Commit e7af4033 ("rpm: use the new
gettext logic") introduced a really nasty bug: by adding
$(TARGET_NLS_DEPENDENCIES) to RPM_DEPENDENCIES, it completely
overwrote the existing value of RPM_DEPENDENCIES, entirely masking all
mandatory RPM dependencies.

rpm is fairly towards the end of the alphabet, and most other
mandatory dependencies (berkeleydb, host-pkgconf, file and popt)
appear earlier by alphabetic ordering. Only zlib was afterwards, but
since file depends on zlib, it was always built before. This probably
explains why our autobuilders haven't encountered a single build
failure.

However, a simple "make rpm" clearly exhibits the failure, and
obviously the upcoming per-package folder mechanism makes such bugs
even more obvious.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>