rsh-redone: make rsh-redone.mk more Buildroot conventional

This commit slightly adjusts rsh-redone.mk to better match the usual
Buildroot conventions:

 * Variables are named in upper-case, prefixed by the name of the
   package.

 * No re-use of one _CMDS definition in another, and instead usage of
   a RSH_REDONE_MAKE_FLAGS variable to pass identical flags betwen
   build and install steps.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>