Use targets/variables w/o `bmakelib.` prefix

[bahmanm]

Currently all targets/variables are prefixed w/ bmakelib..
For example:

foo : bmakelib.error-if-blank(VAR1)
foo : 
	...

To reduce clutter and save some a few bytes, it should be possible to omit the prefix for all targets/variables if you ask for it; much in the same vein as bmakelib.conf.logged.convenience-target

For example:

bmakelib.conf.convenience-targets := yes
include bmakelib/bmakelib.mk

foo : error-if-blank(VAR1)
foo : 
	...

NB: To ensure backward compat and respect user's choice, this should be an opt-in, ie the default behaviour should be bmakelib.conf.convenience-targets := no.