package/luarocks: rework configuration file for per-package directories

package/luarocks: rework configuration file for per-package directories

Currently, luarocks.mk generates a configuration file with hardcoded
STAGING_DIR, TARGET_DIR, TARGET_CC, LUAROCKS_CFLAGS and TARGET_LDFLAGS
values. This is not compatible with per-package directories, where the
value of STAGING_DIR, TARGET_DIR, TARGET_CC and possibly
TARGET_CFLAGS/TARGET_LDFLAGS may be different from one package to the
other.

Based on input from François Perrad, this commit:

 - Changes the Luarocks configuration file to use os_getenv() for the
   appropriate variables. Since the contents of this file is not
   fixed, it is no longer generated by luarocks.mk using a series of
   'echo' but simply concatenated with the rest of the Luarocks
   configuration file.

 - Adjusts LUAROCKS_RUNV_ENV so that the necessary environment
   variables are now passed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>