Reduced Makefile size

Changed ".equ    USED_GAME, GAME_BPEE" to .equ    USED_GAME,
GAME_SELECTION" in main.s so Makefile does not have to filter for bpee
target.
Made single target seeing that bpee, bped, bpeg, bpre and kwj6 rules
shared mostly the same commands excepting the rule name, replacing with
$@.
GAME_SELECTION sed command is handled by variable $(shell echo $@ | tr
a-z A-Z), which converts the target to uppercase.
Single rm command with sh's {} expansion.
Single line error messages.
CURRENT_GOAL definition to filter specific targets. This is used to
display an error when building for kwj6 target.
Seeing that $(encodedoffset) is used just once and within an $(offset)
check, it was replaced on site. Likewise for $(INSERT).
"seek=___pointer" was replaced by expanded variable $($(shell echo $@ |
tr a-z A-Z)pointer) .