Makefile: improve SPARSE_FLAGS customisation

In order to enable greater user customisation of the SPARSE_FLAGS
variable, we introduce a new SP_EXTRA_FLAGS variable to use for
target specific settings. Without using the new variable, setting
the SPARSE_FLAGS on the 'make' command-line would also override the
value set by the target-specific rules in the Makefile (effectively
making them useless). Also, this enables the SP_EXTRA_FLAGS to be
used in the future for any other internal customisations, such as
for some platform specific values.

In addition, we initialise the SPARSE_FLAGS to the default (empty)
value using a conditional assignment (?=). This allows SPARSE_FLAGS
to be set from the environment as well as from the command-line.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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