infra: add comment describing single/double dollar-sign rules

#  argument 3 is the uppercase package name, without the HOST_ prefix

#             for host packages

#  argument 4 is the type (target or host)

#

# Note about variable and function references: inside all blocks that are

# evaluated with $(eval), which includes all 'inner-xxx-package' blocks,

# specific rules apply with respect to variable and function references.

# - Numbered variables (parameters to the block) can be referenced with a single

#   dollar sign: $(1), $(2), $(3), etc.

# - pkgdir and pkgname should be referenced with a single dollar sign too. These

#   functions rely on 'the most recently parsed makefile' which is supposed to

#   be the package .mk file. If we defer the evaluation of these functions using

#   double dollar signs, then they may be evaluated too late, when other

#   makefiles have already been parsed. One specific case is when $$(pkgdir) is

#   assigned to a variable using deferred evaluation with '=' and this variable

#   is used in a target rule outside the eval'ed inner block. In this case, the

#   pkgdir will be that of the last makefile parsed by buildroot, which is not

#   the expected value. This mechanism is for example used for the TARGET_PATCH

#   rule.

# - All other variables should be referenced with a double dollar sign:

#   $$(TARGET_DIR), $$($(2)_VERSION), etc. Also all make functions should be

#   referenced with a double dollar sign: $$(subst), $$(call), $$(filter-out),

#   etc. This rule ensures that these variables and functions are only expanded

#   during the $(eval) step, and not earlier. Otherwise, unintuitive and

#   undesired behavior occurs with respect to these variables and functions.

#

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define inner-generic-package